Sometimes when surprised, we react badly quite so Iam.
Going back to the OP; I do believe we have parents who've given up after being ground down to nothing for some time.
Children who physically abuse their parent(s) may do so because of behaviour witnessed and learned in childhood. The abuser may no longer be living in the family home which leaves the remaining parent, more often than not the mother, left to do her best to raise her children.
She then becomes a victim of her child(ren)'s abuse as well as having been the victim of their father or another male figure present in those children's lives.
In adolescence and older, a child who abuses their parent may not have witnessed physical and/or emotional abuse in the home. S/he may have been raised in a 'normal' healthy family environment and fallen in with a 'bad' crowd, become involved with gangs and/or started taking drugs. The aforementioned can also result in physical abuse of a parent.
Whereas it's important to include in this discussion, children who witnessed and/or experienced abuse in their childhood, it is also important to acknowledge that this isn't always the case.
Imagine after years of doing your best to raise your child to end up being physically abused by that child. Being ground down to nothing because the child that you love, who is bigger and stronger than you are, is manipulating and controlling your life with fear of what they may do next.
Not all who are abused become abusers, and not all abusers were abused.
Do you think you know when you are going to die?


