Tendonitis can result from several different etiologies. Overuse of the tendon during work or with activities is the most common cause of tendonitis. People who work on an assembly line are often prone to developing tendonitis due to the repetative nature of their job. Tennis and golf are two activities that commonly cause tendonitis of the elbow due to the repetative motion at the elbow.
Direct injury to the tendon can also result in tendonitis as can various inflammatory conditions, such as rheumatoid arthritis.
Lastly, aging can make one more prone to developing tendonitis. As we age our tendons lose their elasticity making them more succeptable to irritation and inflammation.