I believe that the John Argue book, PD and the art of Moving, I think it is called, has a section on 'how' to fall. I no longer have a copy, so can't give a definitive quote, but I am sure it is there. Not fearing the fall is important to not hurting yourself.

I rarely fall now, but do not do the things I used to. In hindsight I had two major falls well before dx, both out of doors in exactly the way you describe, was bruised in quite spectacular fashion both times. After those I tried to get my head round what to do, and the Tai Chi classes I was doing at the time helped me immensely. There have been other times over the years, the most recent a year ago - out shopping I managed to 'trip' over the dimpled paving they use at pedestrian crossings here in the UK for aiding blind people.

However the sudden effect of being mid fall without apparently having done anything to be in that position is one I recognise, the art is in landing so you don't damage anything.

A good physical therapist should be able to help you with this. Falls prevention is a major part of their remit.

Lindy