The dad was horny
The mom didn't want to give it up to him. lol. burt should have took a stroll down to Times Square and got his rocks off!! that nutty hysteric of a wife was the pits! no wonder he was a rageaholic!
as for sonny's possession, forget the "father" and his exorcism, and instead have ol' bad ass dad Burt beat the demon spirit outta him!!! He could have done it, gloriously. A grown man almost getting spanked to death by Burt Young onscreen. It would have been a truly defining moment in the history of cinema. Deniro couldn't do it. Kietel wouldn't do it. But Burt could and would do it. Brilliantly!