Two Ways:

Either with someone who has a lot of bitcoin, (I mean a lot) and can use market power to manipulate the price, essentially simulating a pump and dump.

Or manipulate peoples expectations of the market, for example if everyone things btc will drop in price then eventually it will and vice-versa.  If you look at Augmented Phillips Curve for example, inflation = expected inflation, you affect everyone's expectations and see through it to make a profit, Br-exit is a good example.

-Source, Some Macroeconomics research I've done here and there