Peroneal tendonitis is not very common and is an overuse injury of the peroneal tendons causing symptoms on the lateral side of the ankle. Typically there is pain and swelling just above and below that lateral malleolus, getting worse with increased activity.
The cause of peroneal tendonitis is the same as most overuse injuries in that the cumulative load that gets applied to the tendon is greater than what the tendon is adapted to being able to tolerate.
This implies that the exercise activity is increasing exercise loads on the tissues too quickly for the tendon to adapt. The total volume of activity (eg the miles run) may not necessarily be a problem. The problem occurs when there is a too quick increase in the loads (eg an increases in the miles run per week). If the increases are done slow and steady, then there is a less of a chance of the injury and the tendon adapts to those loads.
Those feet that have a low supination resistance are also much more likely to develop peroneal tendonitis. If this supination resistance force is low, then the foot is very easy to supinate. When this is the case, this means that the peroneal muscles have to work much harder to resist that force. this places the tendons under a much greater load and predisposes then to an overuse injury.