I've noticed that it seems to be geographically based. I'm guessing there's some regulatory or law enforcement pressure on Bittrex. In particular, I've noticed complaints from Middle Easterners and particularly Iranians, as well as Russians and former CIS countries. I can only speak anecdotally, but it looks somewhat political in nature to me.
To boot, people who are fully verified are having accounts disabled too, so it's not just a matter of tightening KYC across the board. I haven't heard of anyone from the US getting accounts locked, either.