Fan theories:
its all Nicholas delusion- and theyre both still on top of the dumpster
The Walkers are chewing on Nicholas guts and for some reason are ignoring Glenn.
Glenn crawls under that very slim space under the dumpster

SOMEONE fenced off that alley and put barb wire atop the fence and put a dumpster where it shouldn’t logically be.

SOMEONE blocked off the stairs leading up to that second floor doorway.

( video of Glenns death scene)
https://www.youtube.com/watch?v=Si2oFJqSo8w

SOMEONE may come out of there and save glenn!