mostly due to the way he trains, IMO.
that bouncy, ballistic, explosive, herky-jerky stuff causes a lot of scar tissue and adhesions and whatnot... some time off and soft-tissue work may help him a bit. Ronnie did similar stuff, maybe not to the extent Branch and Johnny do, and look what it did to him over the years.... I'm surprised Johnny still seems pretty healthy.
he still carries a lot of size and muscularity, so I wouldn't say his 'receptors are burned out' or whatever, his main problem seems to be scar tissue and stuff.