I saw this in real life one time. I was hiking up a creek in Northern California and saw what looked like a rock with fur on it. I went up to touch it, and the instant I touched it they all scattered! I literally screamed like a little girl! They were on an overhang above me so some of them fell down into my hair. I've never been so freaked out, and also fascinated in my life. I sat there and watched them a while longer, and after a minute or so, they aggregated again, and sat there, perfectly still. I have read a study on these creatures, they are known as Opiliones, an Arachnid relative of spiders. This behavior is apparently a response to predation. Amazing.