My ankles do that sometimes. Just out of the blue with no reason. Very frustrating and scary especially if on the stairs.

My left knee completely collapsed underneath me, with incredible pain for about 8 months. Had to grab on to something to stop falling. Happened perhaps twice a month. Also swelled with liquid and needed to be drained. The rheumatologist told me to increase physical therapy. We moved to new city and rheumatologist instantly said I needed a complete knee replacement. Successful surgery in three months solved the problem. Worked for me!

My left knee does that
Sometimes I have I may fall

YEAH that y i have walker but people judge me when out with it ive nearly fallen in middle of road when hip does it