It might take nothing more than one summer that doesn't happen. A few years ago here in Fairbanks it snowed fairly heavily right after Labor Day. Power lines were down, leaves froze in place on trees. Odd thing is, it never even once got above freezing again until spring, which took 9 months, and the leaves were still frozen on the trees all that time. The snow that fell right after Labor Day was still on the ground in May. Sure, it's Alaska, but even so it was a little extreme. It was interesting in the eventual spring how the new leaves would grow past the old ones still on the branches.