Colony numbers peak in August and defensive behaviour peaks with them. Here is what actually happens.
A nest that held two hundred workers in June can hold five thousand by late August. At the same time the queen stops laying worker brood, the sugar supply the colony relied on dries up, and the workers become both hungrier and far more defensive. That is the combination behind most of the emergency calls we take in a season.
Knocking a nest down provokes the entire colony at once, and wasps — unlike bees — can sting repeatedly. If the nest is above a doorway, inside a wall cavity or in a loft you have to walk through, treat it as a job for someone with a suit and an extension lance.