Okay, so my theory is that it's all market manipulation and insider trading. If we assume Trump isn't just fucking stupid, he's telling his inner circle "sell when I blow the market up" and "buy when I backtrack" to massively enrich them by gaming the stocks.
There's no way this will keep working, eventually the market will actually crash, but until then they stand to make a lot of money.
Remember: the rich get richer when the market rises and when it falls.