Wow, I can't believe there are other Leon fans here.
As Brady noted, there are too many variables to even guess what happened: how old was Leon? Was Leon farm-grown or wild-caught? What caused his molting schedule to get out of whack? The water quality in the tank was obviously good and Leon got fed regularly without any other environmental stressors. Was the tank too small and it stressed the lobster out? Did Leon grow too fast?
Either way, Leon got to live a life of luxury. His claws were unusable when Brady got him and he rehabbed a fucking store-bought lobster back to health.
RIP, you glorious lobster bastard.