I fall down. It can be when I am standing or when I am walking. Sometimes it can happen completely out of the blue with no warning. More commonly though I can predict it will happen because of the way I feel:
brain fog, very difficult to focus and think straight - much like a hangover
very tired - actually exhausted
no loss of consciousness
So far doctors have ruled out heart, neurological and lung/respiratory issues as well as tumors and epilepsy
Personally I feel as though it may be a brain chemistry imbalance that interferes with the messages to my legs - I always land on my knees and have never hurt myself and I always get straight back up.
It can be 1 event or it can be multiple events over a period of weeks