I'd reach for the insect repellant before the tinfoil hat. Anyone know offhand an insect with a two year life cycle that has some significant stage that peaks around September?

While the researchers have probably looked into possible insect vectors, it's really not that big of a clue. Also I don't believe the suspect species are even known to be insect-borne.

Such patterns are unusual but potential explanations are plentiful. A warm climate is not only good for insects, but also for a virus trying to survive whatever way of transmission is necessary. Maybe there are two virus species that need to interact, or one virus and one other cyclical factor, and their cycles only coincide every other year.

Such a virus mutates all the time, but no, it's not possible this is "a mutated poliovirus". Like you say, it was mostly eradicated, and diagnostic tests and vaccines are designed to catch any reasonable lineage.