From what I understand, it's random. May be related to the mon, not sure. I did get a critical catch on the first ball when catching a raid Machamp once, and then got another one the next day on a CP 10 Sentret, so I doubt it's on a counter - I didn't catch anywhere near 100 mon between the two, maybe 25-30.
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Critical Catch
Does this happen once every 100 throws or completely random? I just got one today and was thinking about counting my throws as this would come in handy for obvious reasons.