Maybe you reacted so strongly because you got confirmation that he was made into an abusive person because of his own horrible trauma, and indirectly this get you thinking of all the possibilities, the what if's -- what if he hadn't been molested, what if he had allowed to flourish as a person, would he have been a good father then? (Not saying this is a good line of thinking BTW, but it's somewhat possible you went that way). Or maybe hearing what your grandfather did to him further validated the fact that there is a real cycle going on in your family? In a "shit got even realer" kinda way? Or perhaps hearing this triggered guilt, which again is a unhealthy coping strategy- some of us are "trained" to feel responsible for their caregiver's unhappiness, so maybe hearing about your father's great suffering triggered this mechanism response?

At any rate, finding out this new story was clearly a traumatic discovery for you - distraction, the "spacey" (dissociation?) and alertness sound like re-traumatization to me.