How do you get Achilles tendon injury?

Over stretching. Most achilles tendon injuries are either from direct trauma or from overstretching of the foot in a dorsi flexion position, (toes going upward toward the shin.
See below. This can occur acutely when the tendon is stretched quickly beyond its capability. This can happen during sports (basketball, tennis, track) are some examples. It can happen slowly with chronic repetitive microtrauma with runners. It can happen if you are suddenly increasing exercise intensity, frequency and duration. Sometimes it is a minor tear or even a bursitis around the tendon.