I've never had it happen on US (but then I just figured out how to find the A fare on web in the first place), but it has happened to me twice on DL.
I'm guessing as Spiff noted that an air marshal got your seat. I think all of US's 737 are configured the same in first, so if it was 737 for 737 swap this really doesn't make sense and the only explanation that I can think of is the air marshal.
All I can say is the gate agent is lucky it wasn't Spiff who got bumped.