Exactly as stated before. Seat assignments are not guaranteed. (In fact, the Conditions of Carriage state that AA doesn't have to transport you on aircraft at all, as long as you reach your destination.)

Sometimes they bump people out of desirable seats to accomodate special needs pax. It is surprising, however, that they did it without informing an elite passenger.

Moral of the Story: CHECK YOUR SEAT ASSIGNMENTS FREQUENTLY!