"Tennis elbow" is also known as lateral epicondylitis. It's most commonly seen in industrial workers. Repeated wrist or arm motions can bring it on. In others, it comes on without warning and with no known cause.
About one-third of all patients with tennis elbow can't identify a cause or reason for their symptoms. Sometimes there's a reason but the patient doesn't connect the activity with the symptoms. It could be a motion repeated over and over at work. It could be related to leisure or sports activities.
Scientists don't really know what brings this problem on. If it is just linked with overuse, then why doesn't everyone using that motion or doing the same job develop the problem? More research is needed to answer these questions before the problem is solved.