Do you know if the gates used by US Airways and UNITED B757s can't take the larger B763? That might have been the issue on wingspan. Both flights had gone and both gates were free. 405 and 406.
Gates 401-406 can all take widebody.
The US Airways flight departed form 404 which is stand 402C, following that departure stands 402L and 402R then in use so 402C was not available.