1. Infection
Infection that causes bursitis tends to be in bursae that are nearer the surface of the skin, such as those near the elbow. A cut on the skin is an opportunity for the bacteria to get in. Most healthy people would not be affected by opportunistic bacteria trying to get in through the skin. However, people with weakened immune systems, such as those with diabetes, HIV/AIDS, Cancer (receiving chemotherapy or radiotherapy), as well as people taking steroids, and heavy consumers of alcohol.

2. Health conditions
People with certain health conditions are more prone to the formation of crystals inside the bursa. The crystals irritate the bursa and make it swell. The conditions that may cause this include gout, rheumatoid arthritis, and scleroderma.

3. Injury
An injury can irritate the tissue inside the bursa and cause inflammation. Doctors say that bursitis caused by an injury usually takes time to develop. The joints, tendons, or muscles that are near the bursae may have been overused. Most commonly, injury is caused by repetitive movements. Below are some examples of causes of bursitis:

Tennis elbow - bursitis of the elbow is often termed tennis elbow. It is a common problem among tennis players and golfers. 
Clergyman's knee - repeated kneeling can cause injury and eventual swelling to the bursae in the knee area.
Shoulder - caused by repeated overhead lifting or reaching upwards.
Ankle - injury to the ankle can be caused by walking too much and with the wrong shoes.
Buttocks - the bursae in that area can become inflamed after sitting on a hard surface for a long time, such as on a bicycle.
Hips - some long-distance runners, and also middle distance runners and sprinters if they train a lot can develop hip bursitis.