Hypothesis 1: Trauma causing damage to tendons

The mare stepped on the foal's right hind leg.  Abrupt change in angulation of the fetlock 
from the weight of the mare may have led to improper stretching of the tendons.  This 
may have triggered pain sensors in the insertion of the tendons and/or in the muscles 
associated with them.  There is also a vascular response resulting in capillary 
hemorrhage, fibrin deposition, and neutrophil, macrophage, fibroblast recruitment.  

Alteration in tendon extracellular matrix associated with repeated and cumulative subclinical trauma or simple overextension of tendon/ligaments result in a biochemical trauma.  This includes active lesions involving muscle action and passive lesions (independent of muscle action).  This then leads to tendonitis.  
There are three stages of tendon response to injury.  The first is inflammation, with the invasion of neutrophils macrophages, and monocytes.  Blood flow increases and edema develops.  Degradative enzymes are released to remove damaged tissue and relatively undamaged functional tendon tissue is also destroyed.  This phase is associated with pain, heat, swelling, and lameness.  This stage is short lasting, only 1-2 weeks.  

Purcutaneous injury, such as lacerations and penetrating injuries can also cause tendonitis.

Horses with extensor tendon trauma can stand flatfooted and bear full weight, suggesting that the injury in this horse is a flexor problem.