So the phenomenon that Alex is talking about occurs when you have a bunch of crabs in a bucket and they're all trying to get out, but in doing so, they, like, wave a claw around and knock another crab down, or they'll try and climb on another crab, thereby dragging it down. The metaphor that Alex is trying to make only works if you believe that the crabs at the bottom of the bucket are somehow resentful of the crabs trying to get out and decide to pull them down. You have to believe that crabs have complex intentions to their actions, which they do not. The actual better metaphor that can be made by the example of crabs in a bucket is about how sometimes doing things in your best interest can hurt yourself.