I've seen three flash floods in my life, all from safe perches. I was never stupid enough to camp in the bottom of a canyon. The onset of this one (the best I've ever seen captured) is faster than any of the three I saw. If those people had been in their camp, they might not have survived. As it was, everybody was fine.
These floods are caused by rainfall potentially far away from the stream bed. The runoff pours into the creek bed 10, 20, or more miles away and just heads downhill. You can have a flash flood in a dry creek bed that hasn't seen rain for 1,000 years. You can also have a flash flood in a dry creek bed that hasn't seen any water flowing for hundreds of years – one of the floods I witnessed was like that, with no water flow in recorded history and no sign of water flow remaining in the creek bed at all.