Simply because they wanna make profits from it if they buy it in a low price.
To answer this question, the reason why peopl sell bitcoib is basically because they want to earn from profit from selling it. And alao they see when the price of bitcoin get high because they could get a huge return from it.
Well this is just one reason and most of the time there are other reasons. One of the most popular is panic selling. When price drops more than %10-15, people get fear and afraid that they can lose everything so they start selling in panic.