It was an SW Motech pannier on their quick release type frames. I’ve a tail tidy so it doesn’t mount off the bike at the number plate which may have allowed the amount of flex to be what it was. The bike also fell slightly downhill as well with a fair amount of force.Begs a question
What was in his right pannier & how heavy was it
Pannier was c.10-12kg at a guess carrying a couple water bottles, clothes etc… with no real rigidity in there to stop the pannier squishing