Some people rationalize. If they see one thing happen, then something else just had to have happened to cause it. When you hear something often enough, you tend to believe it. Rumors abound after many accidents. Workers at a factory, who hear that one of their co-workers was involved in a car crash, circulate all manner of tales.
Some stories are compilations based on several different versions.
Well I heard he was going 90 miles an hour. He paid a speeding fine last week. I was recently in a salvage yard, checking out a severely deformed car when a curious wrecker driver came over to have a look. Just because you have a lot of damage to a particular vehicle doesn't necessarily equate to the speed of that vehicle. Witnesses at an accident scene note such things as damage and relate it to speed.