Springfield is being hit by a huge heat wave, and its so bad its even affecting the town's wax museum. We find Principal Skinner complaining he's found himself "Waist-deep in the cast of M*A*S*H":
As is usual with The Simpsons, the genius writers and animators behind the show included accurate little details for those people who notice such things: we see Klinger's mink stole and Henry's fishing cap among the debris, as well as a pitch-perfect recreation of the signpost.