I tore both of mine about 6 years ago at the same time. Opinions varied, including among my surgeons, but usually it's thought that micro tears in the tendon from tendonitus are a contributing cause. I had "jumper's knee" in both my tendons for basically my entire life and I played three sports in HS, college in baseball and then an absolute shitload of tennis and basketball in my 20s before they ripped in my early 30s playing tennis coming to a jump stop at the net.
You put the wrong force on them and they will snap, especially if weakened. He has a long rehab ahead of him. I was lucky enough to get a great surgeon and I really took command of my own rehab but I had to give up basketball for life. Still play tennis pretty well.
The Yankee kid that made his debut tore his on the electrical box at the Cell. And there have been a couple full tears in the NFL as well. Guys have a really hard time coming back from it. It's worse than an ACL.