My surgeon found excessive inflammation during arthroscopy and excessive bursal tissue in my shoulder is that common and what causes this?

The excessive: bursal tissue is most likely from long term inflammation which can cause hypertrophy or scarring of the bursal lining. The inflammation is either from long term bursitis from constant microtrauma or from acute injury. That being said, its hard to tell without more history. It could be related to other things like rheumatoid arthritis, but if you are otherwise healthy, most likely from above reason