I don't know.  Bittrex didn't respond to our outreach.

I'm sure it had to do with the fork that happened after a miner suddenly pulled 1.2 exahash from Uno.

Now Bittrex joins a long list of former UNO exchanges:

Safecex.com, Shapeshift.io, Btcpool.exchange, Altquick.co, Bitebi9.com, Cryptsy.com,  Coinbroker.io, Coins-e.com,  Allcrypt.com, MintPal.com, Atomic-trade.com, Comkort.com, Coinedup.com, Coinex.pw, CryptoRush.com -- Bittrex