Rude People - What Are They Thinking?

Do you ever find yourself in a situation where someone does something unbelievably rude or insensitive and you're at a loss as to what motivates them?
At a school function the other day, I passed a woman I rarely see and stopped for a brief chat. (She was standing in line for something and on her own.) We had barely spent two minutes chatting, when another mother (whom I didn't know) came up on the other side of our apparent mutual friend, put her hand on the woman's arm, interrupted the conversation and proceeded to talk about something that not only didn't include me, she made no attempt to apologize for the interruption. And then - the first women moved her body to respond, effectively dismissing me and showing no embarrassment at all.
What am I? Chopped liver?
And it happens all the time. I don't know if it's an American thing or just that I obviously move among RUDE people, but at least if I'm the one in the middle, I have the grace to try to keep two conversations going, introduce one to the other or just look torn. Or something.
It's the same as parents whose kids come up and start talking to them when I'm in the middle of a sentence. Most parents don't even say "excuse me", they just turn to the child and start responding. And forget the "I'm talking, please don't interrupt" approach. Some of these kids just keep on with the "Mom, mom, mom, mom", until they wear the mother down. Not too long ago I was having a conversation with a woman (who was asking a favor of me, I might add) and her not-that-young child was pulling on one arm the whole time. We ended up about thirty feet apart and still conversing! No attempt whatsoever to stop the child.
Just what are these people thinking?