I've had bouts of leg paralysis. I have practiced how to fall down so as not to get hurt. Comes on suddenly, though there are a few signs, like weakness before totally losing it. When my grandson was a baby, I practiced a lot, just in case I was holding him when it happened. I don't know the cause. Been happening infrequently for almost 30 years. I used to think it was psychological, as it began after emotional events. Doubt it though.