Haha! I don't know what else to call them. I've had a few experiences like this over the years. It's really not a HUGE deal, so don't flame too hard... it's just conversation....
My 2 oldest girls (8,9) and I were going on BTMR and we got the last 2 rows on the train. It was late in the day and there was practically no line. My daughters always sit together, and I'm by myself. They get in the very last row, and I'm right in front of them. After I sat down and pulled down the lap bar I turn around with a smile about to make a comment or joke or what have you, and there is some bonehead sitting with them! Yea, so there's my 2 daughters and a tween boy (3 altogether) in the back row, and I'm by myself.
Now, I would have not been pleased if he were to sit next to me, but it would make more sense than sitting in a row with 2 people in it already? Like, what the hell?