When a crab is put into a bucket, it will try to escape. As soon as another crab is placed into the bucket, they will start fighting one another and get so caught up in it that they forget about escaping. Basically, to get caught in the 'crab bucket' is to be working hard on something, then one person gets distracted and starts watching YouTube or something stupid, and then another person starts, and then by the end of it, everyone that was working is now distracted and nothing gets done.
"Hey Dude, come check out this video of a shark eating a dog"
"That's funny, hey Bill, come see this"
"That's the shit, everyone, come check this out!"
"And so, the crab bucket begins."