This was a Critical Mass event, which is why the bicyclists are taking up all of the street as a way to reclaim the streets and protest the lack of safety for riders under usual conditions. It’s not legal, but protests are never useful if they’re fully legal now, are they.
The cyclists should pull out guns and shoot the driver in this situation. After all, how can you expect them to do any differently? They suddenly see a driver driving through their parade route, scaring and confusing all of them, endangering all their lives? Sure, they could go around, but we can’t judge them harshly if they just kill the driver or set the vehicle on fire.