An Achilles tendon rupture is caused when the load from the calf muscles exceeds what the Achilles tendon can take and it ruptures. There are two aspects to this – the load and what the tendon can take.
The load is the amount of force that is applied to the tendon from both ends. When the calf muscles contract, a lot of force is applied to the Achilles tendon. This force is greater when the knee is simultaneously extending when the ankle joint is flexing. This can occur in a number of movements in some sporting activities such as tennis (eg reaching for the ball) and basketball (eg accelerating to chase after the ball).
This amount of force is not necessarily a problem if the Achilles tendon can take that load. If it can’t, then the risk for a rupture increase. A well conditioned athlete can increase the load on the tendon to make it more resilient and less likely to rupture. An out-of-condition athlete is at increased risk and so is a tired athlete towards the end of a game. Those over 40 are at greater risk as the tendons are less able to take such high loads. There are some drugs, such as the fluoroquinolones that increase the risk for the tendon rupture. There also appears to be a genetic factor that increases the risk.
In summary, the cause of an Achilles tendon rupture is is a forced sudden contraction of the calf muscle when there is simultaneous dorsiflexion of the foot and extension of the knee on a tendon that is at increased risk or vulnerable.