Grouse, like quite a few of our wild creatures, run in cycles. It's common to have a build up to a high population of them, then a crash to very low numbers.
Biologists will always have an answer for it, usually they will say wet springs.
In my opinion, predators play a major role. When there are lots of coyotes and racoons around, coupled with a very low population of rabbits, it is tough for the grouse.
Couple this with a few mama owls feeding a hungry young family and one wonders how any grouse can survive.