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.
That’s common in the US, but elsewhere theres often parades and demonstrations that se the entire road without blocking them off. Drivers can either wait or backout or be stuck there while everyone goes around them.