There can be many causes of Achilles tendon strain and it is fairly common in runners.

Poor footwear, particularly old trainers and/or poor running techniques can be contributing factors.

Over pronation at the foot (when the foot rolls too much inwards) can create a "whipping" effect on the tendon, causing micro-trauma, which over time can lead to pain and injury.

Poor muscle control around the pelvis, particularly the buttock muscles, can affect the control of the whole leg and may contribute to recurring strains.

It is important with these injuries to purchase good trainers, appropriate for your sport and change them every 300-500 miles if you are a runner.

Try to keep the calf muscle strong and flexible and expect the tendon to take a while to settle.