What causes a reduced arm swing on one side? besides parkinson's disease? what else can cause it?

Many Causes: Depending on the other clinical signs that are present on examination---prior stroke, multiple sclerosis, painful shoulder, pinched nerve in the neck. Need more information to provide you with a better answer.

Many things: Orthopedic issues with the shoulder, and/or neck, and/or upper back; neuromuscular abnormalities; "bulk;" psychological (/"focus") links. Lots of things. Need a good history and exam by your dr or an orthopedist or physical therapist to help narrow possibilities.