One common scenario is this:
1. Cars are approaching in the opposite lane,
2. Truck or large vehicle approaching cyclist from behind. Truck slows down a good distance behind cyclist to allow traffic in the opposite lane to clear the area so the truck driver can move into the opposite lane to pass the cyclist without slowing to a crawl.
3. A second vehicle is behind the truck and can't see the cyclists at all because as the truck slows causing the car(s) behind the truck start to bunch up. Motorist in the first car behind the truck starts scoping out the opposing lane to pass the slowing truck focusing their attention on the OPPOSITE side of the road of where the cyclist is.
4. At the last moment the truck scoots over the center line to pass the cyclist safely.
5. The car trailing the truck has no time to SEE the cyclist and react. (Assuming the cyclist is left of the fog line in the USA)
6. !BLAMO!
This happens a million times a day. It is remarkable that so few cyclists get clobbered.