Because he was hoping to operate his Ponzi scheme for longer, he may still have held some bitcoins for short-term liquidity. Now that he has seen people clamoring for their money, he had to shut down and launder these remaining bitcoins by selling them off.

Needless to say, few "investors" or perhaps nobody will see their bitcoins back. He will now say, I have an unexpected number of withdrawal requests; fulfilling them will take some time, etc. He might even fulfill a few small requests to buy himself some more time. But then it's game over.