I had the same issue.

Sudden e-mail on 24th November and no amount of trying could unlock my account. Spent most of the weekend talking to the Twitter team who advised I needed to e-mail my passport to [email protected], which I did on 27th....called them yesterday (4th) to be told still under investigation and to expect an update in 72 hours.

Wouldn't give me a reason for why the account was locked. Suffice to say, worst time for it to happen as wasn't able to do any online shopping via shopping.ba.com. Doesn't look like my account has been hacked but understandable if they're taking precautions...just doesn't appear to be much urgency on their part.