AFAIK, a lot of exchanges increased the confirmation times a couple of months back due to the 51% attack. Maybe they just never fixed it? Because looking at the trading volume on CMC, it appears that this has been going for a while now.
You are probably right...
This is from Coinbase:
How many confirmations does Coinbase require for ETC transactions?
91,000 confirmations.And that seems big enough. but
595 000 confirmations on Poloniex just seems ridiculous