How do you know they weren't in that seat and got booted out (just like you did now) and you took their seats when you booked your ticket? Maybe they took it back. (Just sayin'. You never know.)

Seat assignments aren't guaranteed and can change for reasons like equipment swaps - or seemingly for no reason at all. All you can do is check back occasionally so if you do get booted, you've got a decent chance at getting your old seats back. But if they're gone, they're gone.