Oh, we'll lookie lookie.  It seems I might be correct except the exchange was Bitstamp.

http://m.washingtonpost.com/blogs/the-switch/wp/2013/11/23/heres-who-probably-did-that-massive-150000000-bitcoin-transaction/

So, I'm speculating Bitstamp did not have the cash and so reached out to other exchanges for an assist.  I doubt a real bank would take BTC as collateral for a bridge loan that size.  Of course, the article posits that the transaction was just a consolidation.  That doesn't make sense, though.  The tag associated with it was "Shitload of money!".  I doubt any exchange would include something like that.  Sounds like someone cashed out and now all deposits and withdrawals are jammed up until those BTC are liquidated.

Perhaps it is a good thing for those wanting to buy.  Price should decline significantly.