What causes one's achilles tendon to tear?

See below: Trauma, cortisone, week end athletes, certain systemic conditions.

Couple of things: Direct trauma or over stretching with hyperextention.

Trauma: Most likely cause. There are medications that can cause the achilles to tear.

Overload.: A dynamic overload of the calf muscle complex, where the muscle and Achilles tendon cannot overcome the stress being placed on it, and the tendon tears.