During BTC's price surge, smaller exchanges either misappropriated users' BTC for short hedging (betting on a pullback) or directly used their reserves for other investments. As BTC continued to climb, the gap grew larger and eventually they couldn't fulfill withdrawals. The so-called Proof of Reserves is nothing more than a performance for many smaller exchanges. There's no independent audit, no disclosure of liabilities, and the Merkle Tree can be manipulated—users simply can't verify anything. So, don't be surprised if you see smaller exchanges quietly running away next. They'll have to swallow the consequences of their actions. If you have coins there, withdraw them ASAP. The best kind of rally is one that rises amidst doubt.