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Hovering is common behavior for male Hybomitra, or at least some common species in North America.

Swarming or hovering behavior is driven by visual cues, some obvious to humans and others not.

Some aquatic Chironomidae and Ceratopognidae look for a dark object on a white background (a rock on a beach, or an entomologist on a beach). Others swarm above the tallest tree, around a tree trunk, or beside a stream bank.

I have seen one Hybomitra hovering next to a 30 cm oak sapling, and another about 1 meter altitude in the middle of a wide trail. I read that Canadian Chrysops look for forest edges.