I think the better question is why didn't you stand up for your beliefs? You can have a discussion without having an argument. I have had civil discussions with a lot of people about carrying and why I do, often because I open carry.
If Grandpa is anything like mine was it would end up in a toe to toe brawl. Some older gentlemen get Pi$$ed just for the sake of it. Sounds like this may have been one of those situations.