I always thought it was implied (at least in the miniseries, I can't remember the book, as I haven't read it since the mid 90s) that Flagg was behind Captain Tripps getting out of the base. There's the crow in the miniseries (which usually seemed to indicate Flagg was nearby) and the fact Campion mentioned the Walkin' Dude to Gary Sinise's Stu Redman when they crash into the gas pumps in Arnette. Although Flagg blocking the emergency door with his cowboy boot was a TAD more blatant![]()
Well maybe there was a reason that the door were a fraction slow to seal when Campion bugged out

(though iirc in the book it was put down to something to do with the electronics).