The placing of the suit jacket is a strange phenomenon too. I have encountered similar types of things. For example, when someone isn't going to poach your seat, but poach your under seat space instead.
A few years ago I boarded a short flight and the middle seat guy had placed his back pack under the seat infront of MY seat. When I asked him to move it he rolled his eyes and whined that he had no place to put it because the bins were full. I told him to place it under the seat infront of HIM.
The more forgiving part of me thought it was simply amateur hour and that this guy had never flown before and didn't know how it works. But the cynical part of me..the dominant part..assumed it was just that entitlement millennials have.
The other incident involved a woman's coat on my seat. She was in the window, I was in the aisle. I got to the seat and asked if it was hers and she said "Actually the flight attendant wants to talk to you about that seat..". I removed it, laid it down on the middle seat, and told her I would wait for the FA. Her eyes got big and she huffed a bit but didn't say anything else.
The FA never showed up and spoke to me of course. The only person that showed up was the lady's husband, clearly disappointed at being in the middle. He then forced his wife to take the middle.