If anyone has a different view, I'm all ears, but I'm finding it hard to see this as anything other than ridiculous. It's as if Farage and Johnson won an election and now that it's come time to implement their policies, they've decided they would rather step down. I don't think they can honestly believe that winning the referendum was all that they needed to do. If anything, winning the referendum was the easy part, and now the real work is about to start. Hard to see these resignations as anything but cowardly in this context. As I said, though, I'm open to other interpretations.
Maybe not cowardly, but selfish for sure. He threw the country in disarray and now he won't responsibly help decide how to accomplish the Brexit tasks and actually perform them. It's kind of like the aircraft carrier "Mission Accomplished" of Bush, but having Bush step down as president immediately afterwards. "Well, I invaded Iraq, it's over, riiiiight!? America! We won, I rule, history accomplished, middle east solved."