Epicondylitis is caused by tensile forces of the forearm muscles, which lead to micro-tears at the tendon insertions on the elbow.
These overloads are usually caused by overextension of the wrist (e.g. when working on a PC with a mouse) or during everyday activities, sports, gardening, knitting, crocheting, etc. and by instability of the wrist, also e.g. when lifting loads and weights, or playing tennis and golf, etc.