Knee problems are common in all sports, but as far as Kung Fu if could be from deep stance training, cartilage break up from years of thowing kicks, damage from a throw or sweep, incorrect or poor stretching, I mean there are a lot of variables and what ifs that can go into a knee injury. And genetics could play a factor too. But back to my original statement, all sports, activities, exercises, ect. knee injuries are common, it's just how it is.
Continuous tension or strain is not good. My uncle laid carpet for several years and now can barely walk from the constant pressure put on him.
"The hero and the coward both feel the same thing, but the hero projects his fear onto his opponent while the coward runs. 'Fear'. It's the same thing, but it's what you do with it that matters". -Cus D'Amato