Nice video Son,
I think it is multifactorial.
The first and obvious point is that the peroneus brevis tendon is in a disadvantaged position squashed between the peroneus longus tendon and the lateral malleolus (fibula).
Other than the obvious unlucky position there are several predisposing factors that leave to compression of peroneus brevis.
Diabetes
High BMI
Biomechanics factors
Smoking
2 weeks ago we had the Canon anatomy day in Melbourne and one of the prosecution cadaver specimens we had was a perfect example,
The peroneus longus tendon was almost round / slightly oval shaped and the peroneus brevis tendon was paper thin and wrapped completely around it. When we lifted the peroneus longus tendon out of the cup shaped brevis tendon we could see a longitudinal split that had formed in the brevis tendon.
It is a mechanical compression phenomenon,
B grade engineering !
I have always said peroneus brevis is a Cheap as Chips, Cunninghams Warehouse quality tendon design.
Steve.