Who is in the wrong here?

All of us were driving along in an 80 km per hour zone.

There was a light about to change. It was green when driver #1 and #2 (that's me) were going through the intersection. It was yellow when driver #3 and #4 went through.

I'm not sure why, but driver #1 braked suddenly. Then they very quickly sped up. I could not tell if anything was in front of them. This caused driver #2 (me) to brake hard and I purposely swerved onto the right shoulder to avoid hitting them.

I looked in my rear mirror and saw driver #3 slow down quickly (of course I couldn't see the brake lights but I could see the gap between us lengthen). While looking in my rear mirror, I saw driver #4 slam into the back of #3.

The first driver was gone. Because I saw what happened in my rear mirror, I did stop to make sure everyone was ok and to be a witness if they needed even though I wasn't hit.

Who is at fault? Who caused this? Which drivers are responsible?