A bunch of green caterpillars on bush?

The problem with caterpillars is that they go "instars" - that is, they grow bigger as they get older. And many times, the smaller instars look totally different than the larger ones. However, the guess of sawfly larvae is quite possible. You can see what they look like here:

http://www.plantpath.cornell.edu/Trees/EuroPsawf.h...

And they do cluster as mentioned. Compare your visual to that image. For future reference, the "upload photo" option isn't working, but you can post an image on a free, public website and post the LINK here for viewing.

Sawfly?

I have seen sawfly larvae act like that. It's weird to see them all react in unison

And yes, when I saw the sawfly larvae, they were everywhere (which is what makes their unison reaction so noticeable)

I can't guarantee it isn't a different bug, but I do know that sawfly larvae exhibit the behavior you describe

Sadly, I come up with 27 possibilities,could narrow this down a bit if you could tell me if it had hairs, spines, nothing, as well as your location.

Yellow with green stripes would be sawfly larvae.

Web worms, the larvae of moths, form in webs.

Twisting a long pole in the web will make the worms drop to the ground and the birds will eat them.

Or squirt them with a strong water hose.