I read that he did not pick his seat beforehand, since he got an upgrade courtesy of AA the day of the flight. I also read that he was trying to move to a window seat just before he was accosted (probably to see his feet better. Also what make anybody think that they can cause a fuel tank explosion at altitude with cold soaked fuel by blowing a hole through the floor.
Definitley not a sophisticated plan with much intelligence, but alas just the appearance of a terrorist will drive more passengers away and acchieve the desired goals.