Crabator is a Predator whose mandible effects don't look convincing. They usually move weird and rest in weird positions. They also usually have so much loose rubbery skin around the mandibles that it instantly reveals itself as fake.
ADI is the worst but still a problem in Predators also.
It doesn't matter though, if the predator takes its mask off and looks wonky and fake then basically it's a crabator. It bothers people like Voodoo and I a great deal and some members here hate that it gets discussed so much.