All it took was for a nearby truck to make a noise or some other nearly unnoticeable (by me) incident, and they would all erupt in flight. For me it was like standing outside a bat cave as the bats came streaming out. More than a few birds whizzed by me, nearly brushing against my head. In less than a minute one would return to the ground, and then within a few seconds the rest would follow suit.
Bazillion Bohemian Waxwings in Flight.
South Burlington, VT
The reason they took flight (next two photos.) This time they did not come back to the crab apple trees as they had done in the previous ~2 hours. Instead many flew out of sight, and the ones that remained roosted on top of high canopy trees. An hour later, I still did not see any Waxwings in the crabapple trees.