You may have been selected for seat assignment by airport check-in staff. I've had that a couple of times. There's no who-ha at the airport, the airport staff don't mention anything. They just give you your boarding pass. I'm not sure why it happens though.
I once rang the airline to find out what was going on and they weren't able to do seat selection either. That's when they said you'll have to do it at the airport. If you feel like sitting on the phone for an hour, you could contact the airline and ask them.