Ugly scene, not something you want to run across on your day ride. Hope all the riders come through OK.
If two and three followed number one in, more than likely it's because of target fixation. Probably the number one reason for motorcycle accidents, especially amongst newer riders. Please remind new riders and practice, practice, practice... looking where you want the bike to go no matter what.
I followed a guy into a Hill Country corner once and just new he was in trouble. Lost sight as he dived into the right hander and out of my view for a second, but the brake light coming on was my clue as to possible trouble. Sure enough I round the corner and he's grabbed a handful of brake, low-sided across the road into a guardrail and the bike is now cartwheeling down the guardrail while he's behind it sliding along the guardrail and, I figured, bouncing off it also.
It was everything I could do not to look at that bike but continue to look through the corner to where I WANTED to go. You see a bike in your peripheral that's cartwheeling and throwing parts as it goes, it's hard not to be captivated by it and naturally want to watch. I knew if I looked I'd probably go the same place. Luckily, this day, the rider got up and walked away with minor injuries.
Target fixation will get a motorcyclist in a second. Remember and practice, look where you want the bike to go no matter what! Pass it on as I'm still surprised how many motorcyclist have never thought or heard of it.