There are many causes based on observation and personal experience. Aging is a common problem with all types of injuries and this is one of them. An aging body simply is more vulnerable and usually by the time you hit your 40s the risk increases every year. I've recommended many aging players to go for more forgiving frames and lower string tension as they age, me included.
Poor technique can be a cause and one-handed backhands combined with a light-weight, stiff frame, with the wrong strings and tension can do it. Tennis ball can cause elbow injuries as well. As a kid, I tried Tretorn balls, only once, and had my first lesson in getting arm problems from equipment. The tennis ball type has to match the surface and altitude.
The Pros job is to guide students in the proper choices to avoid elbow injuries.
