How did this happen? I was kicked out of a gym that went from Valor to Valor?

Odd event at a local gym I was hoping someone could explain. I am team Valor.

I just took the gym and dropped a Snorlax, the only defender.

Walking away, I noticed it's health was down - it was being attacked. Not a spoofer, I saw the guy next to the fountain, who jogged away as I was walking back.

He booted my Snorlax, but replaced it with another VALOR defender, a Rhydon. At no time was it Mystic or Instinct. After the takeover, I was locked out for a few minutes and almost out of power on my phone, so I couldn't add a defender (this seems to be an annoying trait of the new gym system - you can't add a defender right after someone else takes it over).

First, how is this even possible?

Second, why - he could have just added the Rhydon to my Snorlax without a battle.