In most cases, bursitis of the shoulder is caused by performing repetitive movements for an extended period of time, such as throwing a baseball, playing tennis, painting, scrubbing, gardening, carpentry, etc. Shoulder bursitis can also be exacerbated by a physical trauma or previous injury to the joint area. Your risk for developing bursitis increases with age as joint components, including the bursa, tend to decline as you get older. People over the age of 40 are at the highest risk for developing bursitis. Shoulder bursitis can also be brought on by other joint conditions such as rheumatoid arthritis, psoriatic arthritis, gout, and thyroid disorder.