You can't question human behavior because there are too many variable to that question, what did the mother do? Was the child mental stable or if he has anger issues? Did the mother hit him first and he was defending himself, or was he under the influence of Drugs/Alcohol? You need more info to know why.
because they have no self control, i am guessing that the person hitting their mother is a guy, the reason for this is that boys are no longer trained to be men, they are trained to be marshamllows with no self control, an entitlement mentality, the idea that they can do whatever they want, whenever they want, and the idea that screwing anything that walks is just fine. so if its a girl, she is probably frustrated with something and venting, and will regret doing it in a few hours, if its a guy, then its because he has no gentlemanly qualities, or chivalry, and so he thinks its okay to treat his mother without respect, and he wont give a care later