Because there is a psychological barrier that says that this specific stock or etf is NOT worth less that this exact price, so it bounces up. It's just the market dictating that to all.
All we are saying is that because A, B and C occured MAYBE D will occur as well... again.. its a probability of happening and is based on observations made, doesn't mean it is a rule of nature. And it is very different to state something that has occured in the past and deduce something from it than observing something and ANTICIPATING that it could likely occur again and so positioning onself in entering that trade while managing the risk (stop orders). It's that simple.