I've found around here, it's usually something large enough to be a threat but small enough to be only a maybe threat....like the neighbor's cat wanting to hunt mice around the coop. The "threat" is something they want to leave, but isn't quite intimidated by the roosters. (So not a predator like a fox or lynx that would carry them off). It's more like a threat to babies.
Alternatively, it's something they've sensed more than seen so they aren't even sure what is there. The whole flock is on alert until they decide it either isn't a threat or can be categorized into type of threat so they know how to respond.
@Ponypoor give your boys some extra goodies (they'll share with the ladies and look better with them). They earned the treats.