A lot of stuff is Crab Bucket. Being born in the wrong street/shape/race/sex is Crab Bucket. The endless fight for recognition/food/love/appreciation is Crab Bucket. Just ask Pepe. It takes a visit to Verity Pushpram for Glenda Sugarbean to fully grasp the subtleties of meaning.
Anyone as experienced in handling seafood as Ms Pushpram knows that no lid is necessary on a bucket of crabs. If one tries to climb out, the others will pull it back. Crabs fall considerably lower on the evolutionary scale than primates and, certainly, people, so this seems to be a basic force of life. Petty jealousy or a reluctance to see anyone do better has probably slowed the development of civilisation more than anything.
Pepe used the expression "crab bucket" to describe the slippery pit created by this destructive tendency. It has other names elsewhere, like "tall-poppy syndrome" and "envy".