he clearly wasn't trying all that hard to free himself until he saw Carl. the walkers seem to get amped up when they're close to people to feed on. plus, if you've ever gotten stuck in mud like that, it's a vacuum effect more than anything, so pulling harder doesn't do much. seems like he pulled from a different angle when he was horizontal reaching for Carl, so maybe that popped the seal or something.
but the 'sneak attack' was just poor writing. what, was he taking a power nap from his beef meal? he had the itis? stupid. i'd have had him rip a leg off trying to get at Carl (already established as plausible from the well incident), and then have Dale trip over him or somthing instead of him just magically appearing behind Dale like he did. would explain why Dale didn't see him, because he was crawling thru the tall grass.
they've been there for weeks, no secondary fence or anything as protection. just chilling in tents? for a show that's supposed to be about human reaction to a f'd up situation, they sure are screwing up the human reaction part.
the book was far better. the campsite attack from the first season happened, but it was explicit that it was because they got complacent and sloppy. the show kind of hinted at it, with the fire that Shane made the redneck put out in one of the first couple episodes, saying the walkers were attracted to the light, but then they had a bonfire a few nights later precipitating the attack. but in the book, they learned their lesson, and got paranoid as they should. accidents still happened, but it wasn't from sheer stupidity, which only deepened the paranoia.
the prison arc only makes sense from that angle, so the writers better clear that up quick.