Some people sell their bitcoin to make some short term profits. There is nothing wrong with that. Bitcoin has risen in very 3 year period.
I think they make lower order purchases than usual so they want to achieve that, have you seen that after the dump there is always a pump to make a little profit for these kind of people.
Basically the first price starts to be pumped so that this group of people gets a profit.
Then they want to panic to sell lower, they do not make a profit at this stage.