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.
“Dangerous to drivers”
Because you’re going to give yourself an aneurysm over a minor inconvenience? You’re in a giant suit of armor. You could hit every single one of those cyclists without sustaining even a scratch.
If the cyclists were actually dangerous to drivers, do you think the diver would be accelerating aggressively towards them?