Crabs in a Bucket Syndrome, from Nigel Davies
I learned about this for the first time this weekend: If crabs are trapped in a bucket, they'll drag back any of their number that tries to escape so that they all share the same fate.
Stefan Jovanovich adds:
In describing his early days as a scuffler and then a scuffling actor, Michael Caine used the same expression. Among his fellow poor — friends, family and neighbors — "Whenever anyone tried to get a leg up, people would reach their hands — not to help push but to pull him back down. It was like crabs in a barrel."