Why would a cyst form on back of heel by insertion of achilles?

Inflammation: Cysts and bursa are fluid filled pockets that occur due to irritation and inflammation.

Cyst: It's possible to develop a ganglionic cyst of the tendon. But it can also be bursitis.

Irritation: A swelling near the insertion of the achilles tendon can be many things. An exam and possibly an MRI may be necessary to tell what it is and why. A bursa in this area can form over a bony prominence as it rubs against the heel counter, ganglion cysts form near tendons if there is irratation as well or for no good reason. Epidemal inclusion cysts occu r if there has been a cut in the area.