Tendons attach muscles to bone. The apophyses are the connections within growing bones. In this case, we are examining the connecting tissue between the heel bone (calcaneus), the growth plate that lies on the backside of the heel bone, and the Achilles tendon, which connects the calf muscles to the foot.
Because these mechanisms are involved and actively being used with every step we take, they are under constant stress. When the growth plate becomes inflamed and irritated, it essentially expands the apophyses, causing pain and tension in the back of the heel.