It is my guess that there is some unresolved resentment there, between father and son, and probably no one knows what it is except the son. Not that it justifies what he did, but having insanely dysfunctional in-laws that continue to be oblivious to their harmful behavior and live in a constant state of gaslighting, I am trying to give the kid the benefit of the doubt. I hope they can salvage their relationship by really getting to the root of their issues.