What happens to a joint to make it have bursitis? is it a stress injury from overtraining?

Overactivity causes: Over activity of a group of muscles lead to irritation of the bursa which is a cushion between the muscle layers, resulting in bursitis. Trauma, inflammation, infection can also lead to bursitis. Sometimes an inflammed joint can lead to overlying or adjacent bursitis.

Bursa = not a joint: A bursa is a fluid filled sac that allows muscles, tendons to gluide over bone surfaces. It can get inflammed and can cause swelling and/or pain. It is not inside joint. Mnay bursas are located around joints like the knee, hip or shoulder. If the joint is not being used efficiently it can stress the bursas and cause bursitis as can injuries, infections, calcium deposits.