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.
No idea what’s happening? Who doesn’t know what critical mass is? Even if you didn’t, if you’re so cousin-fucking stupid you can’t look at this situation and see that this is obviously an event you shouldn’t drive into, then you should be locked in a rubber room and not be allowed near sharp objects, let alone be allowed to drive a car. You’re too stupid too be allowed in public.