According to a quick google search, stifle problems can be more common in horses with a very straight hind leg, poor muscling, or developmental disorders.
But to my knowledge, the stifle is a pretty complicated joint on horses. My horse reached 21 years old without being lame a day in his life, and as soon as he was not very fit or ridden often, he would stumble pretty often in his hind end. I attributed this to overall stifle weakness; he isn't very straight-legged, and his conformation overall is good.
Toofine - 1998 Half Arabian
Minnie - 2013 Morgan