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Interesting story if true as you've outlined. And I'm going to buy it because you seem sincere. I have no idea of the real answer but perhaps it's pheromones that either the original hornets or new hornets detected?? Is there an entomologist on the Board that can give a definitive answer?

Interesting. The nests of all paper wasps die in the winter and are not reused. All the males die off. What’s left are the “pregnant” females (queens) that start new nests each year

May have been like above poster said and they hatched out

Not familiar with European Hornets but if you ever run across a giant Asian/Japanese hornet the existence of the devil will be confirmed for you.