Because it happens more times than not. If you watch the pre-market activity, it usually tries to either shake out the weak or suck you in. After the first 45-60 minutes, thinkgs get back on track.
Makes sense. The MM want to scare the people on the fence out of the market by imposing their own sell orders. When the weak get out, they come in and scoop those shares up. Why? Because they want those shares. In this business, 0.10-0.20 can mean a lot of money. And they try it every day. The same applies in the opposite direction, if they want to shed themselves of shares.