Poloniex was hacked 2 years ago. They recouped their loses by increasing trading fees for a small period of time so no user actually lost anything from their balances.
http://www.coindesk.com/poloniex-loses-12-3-bitcoins-latest-bitcoin-exchange-hack/
That's actually a commendable action, on their part. Thanks for sharing, as it shows them in a different light!