Probably from kneeling down to sweep up the fallen hair.
My right knee went first, but at the time it seemed as if the left knee would be about 5 years behind it. But then (2008 Feb. 29) I had the right knee replaced, and after a month or so the left knee stopped bothering me altogether. Apparently it had been overcompensating. The right knee’s been just fine for over a decade now, but the doc told me that it wouldn’t stand up to the stresses and torques of basketball, so that was regretfully the end of my hardcourt career.
Ok, I feel for anyone who’s going through anything with knees or any bones of any kind. My body is riddled with metal & my joints are toast thanks to arthritis, but my knees are, so far, are OK. I’m amazed that I don’t have that problem! Knock on wood everyone!!!!!!!