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.

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?

Even if such insect exists, they would have alternating two groups that would appear like annual, like cicadas.

Or appear every 13 or 17 years, like some cicadas. https://en.wikipedia.org/wiki/Periodical_cicadas#Lifecycle

This is a great natural wonder. Thank you.

It also invalidates my logic. Maybe there could be some vector with biannual periodicity.