Generally, it is because they follow the volume . When they are on the ask and can fill and then the volume tails off, they do lower their price. All about cost basis and need. Question for you. If you had 1 mill shares of any stock you wanted to sell, wouldn't you start at the ask and if there is volume you would keep it there, but if volume starts to slow and others want to sell, they move the ask down, wouldn't you? Same with bid.