The main cause of lateral epicondylitis is overuse, from repetitive motions of the arm and wrist. This in turn leads to wear and tear, and weakness of the muscle over time. This is why people in sports that require the use of their arms are more prone.
Activities could also cause tennis elbow. Vocational or recreational activities that use the forearm and wrists excessively can cause tennis elbow. Activities such as carpentry, painting, plumbing, typing, knitting, are risk factors that can contribute.
Age is also another factor. As people get older from 40, Muscles gradually begin to wear and tear. This is higher for those in high-risk activities, and sports that require overuse of the forearm.
However, there might also be no obvious cause for tennis elbow.