Many factors unknown: age, general fitness, past medical/orthopedic history, etc. Association is not synonymous with causation, and all risk factors must be considered. Also, are you spending many hours a week throwing large streamers a mile with a 10-wt rod, vs a few hours a month with dry flies at close range on a 3-wt?
That being said, I personally just do general fitness exercise, some running and weight training, nothing specific to fishing. So far, so good. Mr. karstopo, am I mistaken or didn't you once mention that you are a tennis player or tennis instructor? I would think tennis would be a superb workout for fly casting fitness. Casting wth the opposite hand is an excellent idea, and not just to rest one side. I am sure I am not the only one who has been in a situation (orientation of target water, bushes, trees, etc.) who has said to himself, "Man, this would be an easy cast with the other hand..."