Achilles tears occur in 3 areas:
-Myotendinous junction - usually related to eccentric contraction and acute injury
-Watershed zone - unlike most tendons, the Achilles has no sheath. Instead it has a thin membrane called the paratenon; as a result the midportion of the tendon often becomes chronically hypoxic and heals with excess scar tissue after minor injuries. This results in chronic tendinosis and eventially tears.
-Calcaneal insertion - often due to developmental calcaneal deformity and footwear; but can be seen in athletes as well
Most athletes with ruptures have the second kind. These are chronic processes which is why pros tear it late in their career. Tendon pathology can be accentuated by steroids and fluoroquinolones (e.g., ciprofloxacin) as well as diabetes and other metabolic conditions.
Stretching probably has little effect on preventing this process. Warming up before working out, increasing blood flow, is probably more useful.
Popping posteriorly could also be related to a bursitis, anomalous muscles, posterior ankle impingement or other pathology. Achilles tears occur suddenly, usually without warning, and you'll know if it happens!!