Continuity mistake: Beta kills Marry/Gamma and places her next to a tree. During the scuffle with her, his mask was half ripped and another Whisper comes up to him, now recognizing his face. Beta kills the man, then the camera cuts back to Mary's body, and her head is now turned to the right and her hair has been brushed away from her eye. She also goes from being slouched over to nearly sitting up right against the tree. All this while she's just a dead body against the tree. (00:17:04)
Continuity mistake: After Beta kills Mary/Gamma, he drags her body and sets it up by a tree, dragging her by her hair. There is a jump cut as he walks away, and her eyes move to a different position, as well as the amount of hair over her face changes. (00:16:25)
Answer: The walkers could have knocked them down more than once. There may have been another wave that knocked down the fence earlier and they're going to put it up again soon.
Captain Defenestrator