Causes of Patellar Tendonitis
Patellar tendonitis comes from repetitive stress on the knee, most often from overuse in sports or exercise. The repetitive stress on the knee creates tiny tears in the tendon that, over time, inflame and weaken the tendon.

Contributing factors can be:

tight leg muscles
uneven leg muscle strength
misaligned feet, ankles, and legs
obesity
shoes without enough padding
hard playing surfaces
chronic diseases that weaken the tendon

Athletes are more at risk because running, jumping, and squatting put more force on the patellar tendon. For example, running can put a force of up to five times your body weight on your knees.

Long periods of intense sports training are associated with jumper’s knee. A 2014 study noted that jump frequency was also an important risk factor for amateur volleyball players.