It's not known what causes the fluid to leak out of your joint or tendon, leading to the development of a ganglion cyst. However, women are more likely to get ganglion cysts than men and you are most likely to get them in your 20s-40s. Sometimes, ganglion cysts form after an injury or after placing too much stress on a joint or tendon(eg following repeated stress on the wrist).However, most people have not had an injury and the cause is unknown.
Attribution: BUPA