The phenomenon of a sudden increase in hoverflies is called hoverfly season. This is a common occurrence in the summertime and can be found across various regions of the world.
Hoverfly season follows a pattern in which there are more flies than usual and then they die off at the end of the season. The reason for this pattern is that these insects fly around looking for mates and reproducing.
This phenomenon has been studied by entomologists since it was first seen in North America during the mid-19th century, but scientists still aren't sure what causes this to happen.