This condition commonly occurs in people who perform a repetitive motion with the hands and wrists, however there are other causes of this condition:

Pregnancy
Rheumatoid Arthritis
Post fracture swelling
Trauma to the wrist (Hyperextension injuries especially)
Diabetes
Hypothyroidism

The tendons which run within the carpal tunnel of the wrist can swell, and fluid can accumulate in the tunnel, this leads to compression of the median nerve which runs alongside the tendons in the tunnel. Over time this can lead to fibrous tissue formation on the tendons themselves.

Symptoms can include, aching wrists, pain at night, pins and needles or numbness in the hand, especially over the thumb, index and middle finger. In extreme cases wasting of the muscles in the thumb and hand can occur leading to weakness.