Originally Posted by DXchulo

You have it right. Rider #1 never even made it to the car's back bumper. It looked to me like the car stopped to let them pass, they slowed down and started to pass only after it looked like the car wasn't going to move, and then the car decided to move right as they decided to pass (probably thinking the bikes weren't going to pass, after all). You've probably seen that before at 4-way stops, right? Rider #1 probably pulled an emergency brake (which he had plenty of time to do) and Rider #2 ran into him.

For the record, there were tons of bikes out, but this was just a 2-man group, not exactly a paceline. They certainly weren't moving at full speed at the time.

I ask who's to blame because Rider #2 was upset at Rider #1, which didn't make sense to me. Sure he went down and adrenaline was probably going, but I always thought if you hit someone's rear wheel it's most likely your fault. Since there weren't any riders beside him, at the very least he should have been able to swerve out of the way if he couldn't brake in time. He told Rider #1 to signal when slowing. I wonder if Rider #1 just stopped on instinct without having time to shout that he was slowing.

I'd give some blame to Rider #1, but most to Rider #2. They probably should have stopped behind the car and waved it on.