Feet-Hurt.Com

The most common causes of Achilles tendonitis are poor training techniques particularly by older athletes. Inadequate warm-up, a rapid increase in mileage, or rapid increase in intensity of other sports will put excessive tension on the tendon. For runners the type of surface used for running can have a negative impact on the Achilles tendon. Concrete surfaces do not allow for any shock absorption on impact and thus the shock of the foot hitting the ground results in damage to the posterior musculature including the Achilles tendon. Running on uneven surfaces such as sand or a “banked” road results in more expenditure of energy by the lower leg muscles making the Achilles tendon more prone to injury. Running uphill applies more strain to the Achilles tendon increasing the possibility of injury.

From this site:  http://www.foot-pain-explained.com/achilles_tendonitis.html