Continuity mistake: The hole in the wall Carol makes with the knife grows, depending on what angle we see it from.
Continuity mistake: Daryl and Carol are walking to an abandoned home. Carol walks by a bush with bright sunlight coming from the left of the shot. The angle changes and it's overcast with no sun. Then we see Daryl again and it's once again sunny.
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