There are two major tendons on the outside (lateral) portion of the leg. They are the peroneus brevis and longus. They run through a sheath that keeps them locked into place. If this sheath becomes damaged, the tendons can snap out of place and then return to their original resting spot.
Popping and snapping can be caused by a few different problems. It could be soft tissue (most likely a tendon, ligament, or muscle) that has been damaged or injured in some way and has become larger than the space around it allows. This could cause it to rub up against a bone, producing the snapping or popping sound you hear.
It could also be articular cartilage that has become torn up or has broken off completely from the bone. Articular cartilage acts like the brake pads in your car, except it’s for bones. It protects bones from rubbing on each other, and over time can become worn down, or even damaged. If the articular cartilage catches on the bone or breaks away and is stuck between two bones, it could cause a snap or pop to be heard along with a feeling of catching, locking, or giving way. Generally, popping and snapping is not normal, and it may be necessary to visit a physician or orthopedist who can help you figure out what the problem is and how to treat it. (This answer provided for NATA by the Eastern University Athletic Training Education Program)
More Answers from National Athletic Trainers' Association