I don't think you do know the answer & I don't either. If the
vet has given Rocky a clean bill of health and you haven't
changed food or feeding times, it would seem to to be an
emotional/behavioral problem. This sounds like a question
you should ask of a dog behaviorist. Healthy dogs don't
suddenly start a new behavior, unless they are under stress
of some kind. I would either seek a second opinion from another
vet(health wise) or get in touch with a dog behaviorist about
this problem. I don't think crating the dog will fix his problem.
I hope you can find help for Rocky.