What might have happened is that while you were gone, other cars came and occupied some of those 15 other parking spots at least one spot away from each other on both sides. So it doesn't really take a lot of cars to see that wherever someone parks they'd park right next to someone else anyway, so the logic to some driver might be, let's just park it to the spot nearest the entrance for convenience's sake. Or if you weren't near the entrance to begin with, you were just unlucky the driver picked to park next to you 'cause there weren't other parking spots that he could park away from any other car. So when other cars leave, it looks like the driver parked right next to you when there's plenty of free space.

Or at least that's what I tell myself 'cause it also pisses me off too.