It's usually caused by a break or sprain. I broke my ankle and I couldn't do physical therapy because my ankle swelled and turned odd colors, but my orthopedic surgeon said the break was totally fine. Basically it broke and then the brain never knows to turn off the pain signals, so it sends them other places.

I ruptured the fascia on the bottom of my foot. I had some small tears and bruising. I moved too fast and it snapped. It was immobilized for so long and was so painful. That is what caused it with me. I read that it can occur without injury also.