Most of it is probably just normal statistical variation, but a low/moderate ENSO index (which we had over this summer) is actually associated with somewhat lower average summer temperatures in most of the United States. The North Atlantic Oscillation, which is a similar quasi-periodic cycle in (shockingly) the North Atlantic, also tends to cause somewhat lower temperatures on the East Coast when it is in its negative index phase, which (again) it was over this summer. Both these influences are dominated by inter-annual variability on smaller scales, though.