My conspiracy theory is Bitstamp needed to buy back the coins they lost to balance their books and the best way to do that was crashing the price.
The price on January 4th was around 285 USD and they lost ~18K BTC or ~5.1M USD at the time. They decided to cover the lost in BTC for their customers, they didn't have any option but to absorb the loss and respect the BTC balances of their customer in the exchange accounts.
But did they really had all those coins? Or were their books unbalanced and operating on fractional reserve, so that if everybody withdrew at the same time they wouldn't be able to honor all of the coins. And even if they did, how to recover their losses?
Answer: crash the price. At the bottom of the crash on Bitstamp 152 USD, getting the 18K back was ~2.7M USD, that is almost half the amount of the original loss. Obviously I am just speculating, but they definitely benefited from the crash, that conveniently happened right after they "lost" all those coins.
This unregulated market definitely needs a decentralized trust-less exchange, so we don't have to guess.
As good a theory as any.
For the 238th time, every idiot who keeps a balance on an exchange is contributing to the downfall of all cryptocurrencies, because the exchanges just short it all to oblivion or rig the markets at will so they can extract fiat from your increasingly worthless cryptobag.
Currently all the exchanges are just acting as central banks. We know out there in the real world how that works out, and who profits. Hint: not you.