Can anyone explain to my why this is resignation-worthy?

Simon Danczuk, Labour MP for Rochdale, ... told the Mail Online it was “like the Labour party has been hijacked by the north London liberal elite, and it’s comments like that which reinforce that view”.

The comment was, “Image from #Rochdale.” It was a picture of a white van outside a house covered in English flags. And that can drive a shadow cabinet member to resign. What?!?