Doctors don't know why ganglions develop. In some cases, the wrist has been injured previously. Repetitive injuries, such as those that can occur from playing tennis or golf frequently, seem to play a role in ganglion development as well.
Doctors also don't understand exactly how ganglions form. One theory suggests that wrist ganglions are formed when connective tissue degenerates, or is damaged by wear and tear. The damaged tissue forms a weakened spot in the joint capsule--just like a weak spot on a car tire that allows the inner tube to bulge through. The joint fluid may escape through this weakened area and begin to collect in a sack, or cyst outside the joint. Over time this cyst grows larger. The joint fluid seems to move out of the wrist joint into the ganglion, but not the other way. In the end, a clear, sticky fluid fills the cyst. The fluid is a mix of chemicals normally found in the joint.