Hermit crabs form gangs to steal other hermit crabs’ shells
There are 1100 species of hermit crab, most of which have an asymmetrical abdomen. They aren't born with the shells that protect their abdomen, though, hence the "hermit" in their names. Those are actually empty gastropod shells that they find.
From time to time, a hermit crab has to replace its shell with a larger one to fit its growing body. When a hermit crab is without a shell, it's pretty vulnerable, so they try to find a replacement as soon as possible. This isn't always easy, though.
Gastropod shells are a limited resource and hermit crabs often fight over them, sometimes leading to the death of a competitor. This only happens when the crabs are of similar size, of course.
Hermit crabs often "gang up" on a hermit crab with what they perceive to be a better shell, where they will actually pry its home (shell) away from it and then compete for it, and one will ultimately take it over.
Several hermit crab species, both terrestrial and marine, use vacancy chains to find new shells; when a new, bigger shell becomes available, hermit crabs gather around it and form a kind of queue from largest to smallest. When the largest crab moves into the new shell, the second-biggest crab moves into the newly vacated shell, thereby making its previous shell available to the third crab, and so on.