Why?

I know this is petty but me and old people really don't get along. Yesterday I was at the grocery store waiting in line buying a few simple damn thing. I had in front of me a old hag who didn't know how to use the debit swiper. How hard is it to use it? What couldn't she understand about if she wants cash back or not? Then I had soccer mom trying to buy something that wouldn't scan so they had to do a price check. Turned out the damn thing wasn't for sale and was for display only. Then she tried to negotiate to try and get it for free. Now this is the new anger free bastard. Normally I would just tell her to get the fuck out of the line. But I didn't. I was so proud of myself, but then Mr. 100 year old Father Time behind me starting putting his stuff on the little revolving black thing on top of my stuff while the cashier was ringing up my stuff. The old man's wife was with him telling him that he is putting his stuff with mine, but he said he didn't care about this and I quote, "asshole in front of me". Why? I turned around and pushed half his shit off the conveyor belt. I laughed and told him that I guess I am really an asshole. I purchased my stuff and left. I don't think I'll go back their again. And yes I know I should've picked another line but you never know how long it's going to take. When stuff like that happens I always say to myself, it could've been worst. I could've went psycho and body slammed the old man while destroying the promotional display.