Kind of like when a school of fish all turn the same direction?
Mmmh...I think fish follow a different principle. More of like "stay with the group".
There once was an experiment where they put a bunch of people into a big hall and gave them one requirement: Never walk alone, always stay close to other people. After a while, they ended up going in circles like a giant ant death spiral (though I think the ant death spiral is another phenomenon involving pheromones). Ant Death Spiral
I think if you had a "predator" or something scary attack that spiral from the side a couple of times, even the humans would have quickly turned together just like a fish swarm (maybe it would take a handful of "attacks" to make everyone catch on that you'd better turn with the group because they might see something you didn't).
But maybe that's all it takes: An involuntary "spasm" by a baby spider (or five) in a good place - and all the other spidies follow. So that is
sort of like a turning fish swarm.
I just really wished I understood the benefit of twitching spiders. Maybe they want to appear as one big thing rather than a bunch of little things?