The main cause is frequent and repetitive movements of a part of the body - for example, typing, using a computer mouse a lot, etc. Other factors may contribute, such as poor posture whilst doing the movement, using excessive force whilst doing the movement and not having enough breaks from the task.
However, the precise reason why RSI develops is not clear. In many cases there is no swelling or inflammation and there are no other obvious problems which develop in the muscles or tendons, and yet symptoms develop. Also, it is not clear why some people develop RSI and not others who do the same repetitive tasks.
Research suggests that psychosocial workplace factors (which usually means stress at work) can also contribute to RSI. It may be that stress increases muscle tension and/or affects how the body feels pain in general.