Walking down steps - knee problems

A week ago I was descending steps at the library and was not using the railing to assist. It was rather disconcerting to have my left knee jut out to the left side as I put weight on it. It started hurting and regardless of how much I concentrated and slowed down I couldn't get the knee to stay in alignment. I never did figure out the exact cause but if I use a railing the knee works fine. I'll have to experiment a little more.