Most common cause of insertional tendonitis of tendoachilles is -
Achilles tendon injury

Achilles tendon is the thickest and strongest tendon in the body and is also the one of the most commonly injured tendon in the body.
The most common Achilles tendon injuries are :-


Achilles tendinosis (formerly called Achilles tendonitis)
Achilles tendon rupture

Achilles tendinosis (Achilles tendonitis)

Achilles tendonitis is a condition that develops over time and is associated with degeneration of the tendon and inflammation that causes pain.
Achilles tendonitis is described as insertional or non-insertional:-
Insertional achilles tendonitis :- Occurs where the achilles tendon interts into the calcaneum, i.e. at the tendon-bone interface.
Non-insertional achilles tendonitis:- Occurs about one to two inches (2-5 cm) proximal to the site of insertion of the tendon.
The etiology of Achilles tendonitis is overuse.

"Achilles tendonitis is a chronic problem; it occurs primarily from overuse".  — Healthguide for sports
"The etiology of Achilles tendonitis is overuse".  — Essential sports medicine

Achilles tendonitis occurs more commonly in Atheletes, joggers and hikers.
Overuse
Improper shoe wear
Runners and jumpers
Steroid injections