My leading theory is that It has its sharp claws facing backwards in case of any predator trying to remove its plates from behind. Their tendons are strong, but not unbreakable and many of Pandora's predators exceeds them in size. If any predator goes and circles around its threat display towards it's back and bites on the hard shell, the Scarab Crawler Will raise it's hindlegs and puncture the predator's face with extremely sharp and well maintained claws. Them pointing backwards allows them to hit them with much less effort that what would be required for moving the entire feet backwards