I think it's just a knee-jerk instinct that says 'nooo, I don't want to be involved, don't ask me!' but I was there, I saw what happened, I was the only person out of the many present who bothered to stop and see if anyone needed help, so I figured leaving my details was the right thing to do. It's just a bit worrying to get drawn into someone else's problem - I don't want them relying on me and my poor memory because I really wasn't paying all that much attention! They did manage to say thanks at the time, despite being so shell-shocked - and certainly latched on and clung to me until the ambulance arrived! I was just glad I had my sensible head on that night.