Probably a dead animal nearby. Roadkill or something.
While I was searching for their range I found a local thread about it from 2014. Apparently it's a longstanding roost that they use every summer. Some NIMBYs back then cut down a third of their trees to build on their lot and it was only the city's urban forest that kept them around. Urban Greenspace Gang Urban Greenspace Gang 
Some NIMBYs back then cut down a third of their trees
I nominate a corpse for them.
Thats really neat. Thats a lot. Theres a place where I go camping sometimes along a river and sometimes the turkey vultures do this too, but not in the amount seen here, maybe 10-15 at the most. I wonder if they are roosting or something. I see its still pretty bright outside there so maybe its not time for bed. Maybe its just a party and your invite got lost. Sometimes I wish I knew more about the natural world around me than what I already know.
I figured they were feeding or something, but there wasn't a corpse or the smell of death in the air. Apparently this is a historic roosting site going back at least 12 years.
They are around me a lot, though usually just 3-4 at a time. They are cool birds. They are fairly large. It's cool how they fly around and coast in the wind.
I found a YouTube link in your post. Here are links to the same video on alternative frontends that protect your privacy:
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