What causes patellar tendinopathy?
This disorder is generally agreed to be an 'overuse' problem, with the tendon developing frequent small tears which it attempts to heal. Instead, the tendon develops a mushy ('mucoid') degeneration, which weakens it, causes local discomfort, and may predispose the tendon to rupture.
Confusion of names
The condition used to be called patellar 'tendinitis' or 'tendonitis'. The term '-itis' in medicine means 'inflammation' - a process of irritation, increased blood supply, and infiltration of the area with special inflammatory cells - but these features are not actually seen in patellar tendinitis. So there is a lot of medical literature suggesting that the condition no longer be called 'patellar tendinitis' but 'patellar tendinopathy' or 'patellar tendinosis' instead, implying a degenerative condition rather than an inflammatory one. For your research, you need to search using all of these keywords.
Patellar Tendinopathy. Schwartz A, Watson JN and Hutchinson MR. Sports Health. 2015 Sep; 7(5): 415–420.
Current concepts in the treatment of patellar tendinopathy. Reinking MF. Int J Sports Phys Ther. 2016 Dec; 11(6): 854–866.
Patellar tendon: From tendinopathy to rupture. Rosso F, Bonasia DE, Cottino U, Dettoni F, Bruzzone M and Rossia R. Asia Pac J Sports Med Arthrosc Rehabil Technol. 2015 Oct; 2(4): 99–107.