I really don't understand why you are asking why he did it when there's been tons of instances with guys charging the mound without any reason other than they got plunked, looked at the pitcher and snapped. These 2 have a history of going at each other, and Quentin clearly didn't give a shit whether he meant it or not. He's been hit a shit ton, but it's similar to someone getting road rage after getting cut off even though it happens to them on a daily basis, you just snap without thinking at all.