Taking gyms down only a level or 2?

I don't know, maybe they realize that it's going to take too long to take it down? I usually don't see this happen, that's why I'm often saying that it's not that important where your Pokémon places in a gym by its CP. At least in my town, if someone attacks a gym it's gonna go down all the way (Unless their battery dies or something I guess). But then again we don't have gyms here with 10 Snorlaxes...

Well anyway, I don't think there's any specific reason for that. It could also be that they just want to give it a go and see how they fare against the gym. Also one issue could be running out of potions/revives.

I think there are many posible explanations, some examples:
- People are just around with friends/family and take out their phones to play a bit.
- Someone that starts taking it down and does not have more potions/revives to continue
- Someone starts taking it down and sees another gym with an empty space close
- Hackers that start taking it down and lost connection or got banned or any other things.
- Someone taking it down inside of a car and has to leave for some kind of emergency.
- People that just want to take few levels from a high level gym to send a message to their other team fellows that the gym is now more vulnerable, expecting other mates to finish the job

Etc...

I've done it a few times to kick out a specific player from the gym and leave it on a hard to prestige poke. Maybe that's the case.

Maybe spoofers clear the spot for the pokemon of their real account

I observed some of these here. They're casuals doing Pokemon fighting, not hardcore players going for coins.

Before I do my daily 5-gym run, I'll check the road condition - if any gym is pretty high level, I'll take it down below 2000 prestige first (so after I start, I'll only need to do it one more time) before I start. I think it'll give me more odds to succeed a 5-gym bonus.

Nevertheless, I do have witnessed such behavior from time to time, and they are not doing the "Road Condition Checking" as I do; they just do it for other reason.

One of my friend from another team, once actually went into the gym being attacked just to peek. There was actually a Pokemon attacking the gym.
His theory is, it's Team Rocket - An easter egg from Niantic.

Maybe the gym is on a restaurant or store, someone goes there to buy something or get stuck on a line to get a table on the restaurant and the time of wait he or she attack the gym just for fun during 5 or 10 mins of waiting. I do this a lot on stores, but I always finish the restaurant ones.

Taking down a level 10 gym by yourself is an arduous task. Prior to giving up on my team (because allies will use alternate accounts to boot out the bottom of level 10 gyms and replace with their own, so what's the point of trying to hold a gym...) I would do this occasionally.

There's a key in your post to make this work - "they take it down so there's only like one more battle until it's down another level".

If you just barely kick someone out, the first person on that team who walks by will train and put a pokemon in. That's a minimum of 3000 prestige on the gym that never needed to be there. Take it from 10 to barely 7? They now would need to train 7000 prestige to get to 8, something most people won't do.

I used to do this to the gym right near my place. It wasn't commonly contested so I held it at level 10 for my team for a few weeks. When an enemy team took it down I'd take it back every day because it's literally across the street from my house. I left for a weekend and an enemy team had it at level 10, so I'd take it down a few levels when I had time and allies would be more likely to target a level 7 gym without me needing to do all the work. If you don't do enough though, you'll just let the holding team replace the weak pokemon you kicked out with much stronger ones.

I take 7 gyms every day in the morning early. When they get taken back later the same day by other teams, rather than retaking them, I knock them down a level or 2.
I don't take them down all the way, because I have already collected the coins, and the other opposing team may finish them off.
If I see a small Pokemon I always knock it off to prevent quick prestige moves.

Sometimes i attack a gym when i'm waiting for my bus. When it comes i leave the gym no matter the level it is. I know it's weird but i like the battling. ^^

I actually do this quite a bit myself. My reasoning is pretty simple really.
Most of the times I am going to do this is because I cannot currently collect, but I am planning for later. Most people who see a level 8+ gym are going to be deterred from trying to take it down. That is why, in all but the most populous playing places, some of these gyms will often last 3-4 days. But let's say I can cash in 2 hours. If I put in the initial "hard work" of taking the gym down to a 5 or 6, then that pretty much is an invitation for others around to come finish it off. They see a weaker prey, and some people need to collect, and they are now less intimidated by taking it down. So, I can go out and continue playing, catching and hatching, and often times by the time I am ready to cash in the gym will have already switched hands, or is at least at a lower level. And, even if it's not, well, it's right where I left it. It's now in an easier position for me, and I do not have to go through the monotony of taking down a level 10 gym by myself in one go. In other words, it's a little teamwork. Some people can't realistically take down a level 10 gym, so I start by helping them out, and then often rely on someone stopping by and consequently I get a little help in return. Obviously, it's easier to take gyms down together, so I am just sending out an invitation. As far as I know you don't get any particular bonus for being the one to start it or to finish it off, so it's all the same. Just a more efficient use of time.

If I'm walking to work I'll sometimes just knock an opposing team's gym down a level (can often do it in the time it takes to walk past). If I'm waiting for a train I'll do the same. If I've collected my dust/coins in the morning but I pass gyms later in the day I might drop gyms down a level or two to hopefully make a full takeover quicker the following morning.

Personally, I refuse to let an enemy gym stand around at level 10. Might not have the time to fully take it down at a given moment, but it sure as heck ain't gonna be left alone at 10. Sends a clear message that the gym is on constant alert and isn't gonna be a simple coin dispenser.

If you take a level 10 gym down to 32-34K it makes hard to prestige up especially if that last mon is a tuff one. Plus you will need 6-8K in prestige before you can put a mon in, this is what i call crippling a gym. Typically a spoofer will take the gym down to 29K and put a Rhyhorn in at the bottom. Spoofers do this to bait fellow team members to come and train their gyms. If you take it down to under 20K its very easy to win 2 battles regardless of who you fight. So a spoofer can train the gym and effectively set it up for clan members to come build it back up. By taking a gym to 32-34K it forces the spoofer or rival clan to put in more effort. When fighting a spoofer a legit player has to drive out to the gym a spoofer just takes the phone out of their pocket. So legit players are at a disadvantage before they even attack the gym. By crippling a gym you can force the spoofer to do additional tasks to make up for the driving out to the gym. When the training prestige max was 500 I tore every gym down to the ground but with 1K prestige max that strategy does not work. A spoofer will just take your one mon down and have 3 easily trained mons in. Essentially you be will running in circles. Hope that makes sense.

There is a larger strategy to this than you see. most trainers will not attack a level 10 gym by them selves, but if you knock it down a few levels it makes it more attractive to other players to attack it.