I think Dale messedup or made a mistake looking infront of himself 97% of the time, so that those zombies were all upon them as we saw. Furthermore Rick himself was looking behind just as he handed the binoculars to Dale. I think Rick realizes this error made by Dale as he looks upon him on the RV(propbably thinking you should have seen them coming!). Those Zombies must have been inside the woods too, for them to appears all of sudden like that in close proximity to the group.
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