[bonyt]

I’ve heard the line between collusion and conscious parallelism, viewed from the outside, described like this: A group of people leave a building and all open their umbrellas at the same time. If it’s raining, that’s conscious parallelism. If it’s not raining, that’s collusion.

[fauigerzigerk]

This explanation misses the key point almost entirely. Obviously, if there is an external reason for everyone to do the same thing, it is probably not collusion.

But there doesn't have to be an external cause for it not to be collusion. If all kebab shop owners in a neighbourhood charge exactly the same price for a kebab, because each of them has decided individually that charging more or less than the others would put them at a disadvantage, then that is not collusion.

Everyone opening their umbrellas when it's not raining doesn't have to be collusion either. It could just as well be fashion for instance.

Collusion requires an agreement to act in a certain coordinated way. Drawing the line is difficult when there is a silent agreement based on tradition or fear or whatever.