Why do people run their mouths when something doesn't concern them?

I was at Disney world the other day walking in front of a man with a manual wheelchair and he slammed into the back of my leg, causing me to bleed. His wife tried to make him apologize but he said it was my fault for walking in front of him. Alll I said was that it really hurt and tried to hobble off. Then some lady who hadn't even seen it happen because she was in front of us turned around and started screaming about how rude I was. She didn't know the old man or anything either. So, why do people insert themselves into situations that don't concern them and they don't know anything about?