a large gathering of birds that fly across the runway just as you start your takeoff run.
Co-pilot, "BIRDS 12 o'clock high!"
Pilot, "Oh, Shit, we're headed for a cluster flock....."
When multiple dickheads flock into a certain, place, fandom or hobby, transforming it into a clusterfuck of assholery. They will then anally rape it until it is sore, rotten and a genuinely bad place for everyone.
Joe: Did you see what the SJW invasion did to Marvel Comics?
Rex: Yeah. Once they began cluster-flock to it, it was only a matter of time.
The process of geese have sex in a large group, diverged from "fucking".
Dude did you see those geese?
Yeah, they were cluster flocking!