Fear is the major reason, the amount of fud circulating the market is too much for some investors to handle especially those that are new to the game either because they fomoed into investing in bitcoin or they're not mentally capable of handling the pressure the market presents. They simply want to take the easy way out the very moment they faced such challenges and that's done by letting go of the coins the currently hold.

Bitcoin volatility is also very high, you can see the amount of billions the market losses within minutes of a crash. Those billions are combination of investors funds and some have their in thousands of dollars. That's huge loss to handle and only those with experience can handle such loss without doing something out of panic.