Everything is cyclical. The high flying, explosive offenses predicated on chunk pass plays have led to the re-emergence of defenses like Cover 2 (two high shell) and to a lesser extent Cover 4 (quarters). NFL schemes are complex, but generally those two coverages are poor against the run while Cover 1 and 3 are much better against it.   The upshot of the big uptick in Cover 2 is that offenses have been limited on deep shots and have gone even more to an underneath game than previously.