Not that this will answer the specific question of what happened on that specific flight, but...

One time I was flying IAD-ATL, getting to the gate just as the incoming flight was deplaning. My sister, of all people, was getting off the incoming flight. (I was in DC, but had not called her. Turns out, she was in ATL and had not called me.) We sat down at Ben and Jerry's and started to chat. Next thing I knew, she told me she just heard the final boarding call for my flight, so I got on the plane.

I get to my seat, 2C, and my friend Jerry was sitting there. We looked at each other, as neither of us expected to see the other, and he said, "What are you doing on this plane?" I countered with, "What are you doing in my seat?" He pointed to the guy in 2D and said that guy was in his seat, so he just assumed that the guy was confused and took the aisle seat. His wife and child, he pointed out, were in 2A and 2B. (I had not noticed J*** or M*** across the aisle.) The guy in 2D said, "Well, it was worth a try. At least I got a free drink." With that, he took his cup, got out of the F seat and went back to coach.

Jerry said he wasn't sure which he was more upset about - that the guy had the nerve to try to scam a seat in F or that he (Jerry) had never thought of doing that before!

So maybe the first guy in 1A was someone who was taking a chance and, at a minimum, got a free drink before the plane took off.