Factual error: Season 5 and they are still driving cars around! Gasoline won't last years unless specially treated.
Other mistake: Most if not every time someone kills a walker in close proximity or repeatedly bludgeons one that is already dead with a blunt object and there is a CGI blood spatter effect (as opposed to a practical blood spatter effect), the blood magically never gets on the person's face.
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