This video might have been recorded somewhere in Canada, where Mayfly swarms are not uncommon from May to July.
More info:
"Dead mayflies are caused their extremely short adult life span by which lasts only about a day, as they lack functional mouthparts and cannot eat. After emerging from the water as adults, mating, and laying eggs, their sole purpose fulfilled, both male and female mayflies die naturally, leading to large swarms of dead insects accumulating in natural areas and near lights.
Causes of Death:
Natural Lifespan:
Adult mayflies live for a very short time, sometimes only 24 hours, just long enough to mate and reproduce.
No Mouthparts:
The adult form of the mayfly lacks functioning mouthparts, preventing them from eating or obtaining nutrients to sustain themselves.
Reproductive Cycle:
Their entire adult life is dedicated to reproduction. Once they mate and the female lays her eggs, they have no further purpose and die.
Why they accumulate in swarms:
Mass Emergence:
Many mayflies emerge from the water simultaneously, creating a massive swarm.
Attraction to Light:
Mayflies are strongly attracted to light, which causes them to congregate around buildings and artificial lights at night.
Natural Piling:
After their brief life, the dead bodies of countless mayflies pile up in huge numbers on roads, under lights, and near bodies of water, creating large collections of dead insects."
