This is the first time I hear this, and for what I know, bittrex is considered to be one of the better exchanges, yes? Have you tried contacting their support? Usually if the support isn't answering that's a red flag, otherwise might be some bug or maybe some policies you didn't know about.
Their terms and conditions state that they have the right to terminate any account, for any reason, with or without notification and to hold the wallet amounts untils those amounts can be verified to their standards. I actually have heard this a couple times along the line, but never from those that have used the exchange over the long term and not looked suspect.
