Its a joke on the fanbase taken form in comics.
As for the source, the initial appearance was in the ultimate fantastic 4 story crossover where they lead you into thinking the ultimate f4 was gonna meet up with the classic squad. Turns out it was a hoax to get the zombie f4 access to other worlds. As fornthe comic itself, the sentry crashed out of the sky and the avengers at the time rush to the scene. As they find out, sentry was zombified and eating people on the set. The avengers step in and get manhandled and turn zombie and proceed to fukk up anything near by