How do I get the anchor point to stay in the center of a shape group, so it spins in place?

So I have a bunch of simple 2D shape groups that are being repelled from another object. As they fly away, I wanted them to spin on their Y axis. I added a "spin" behavior to one of the shape groups, and changed the spin axis to Y, but instead of spinning like normal, it started revolving around this invisible center point. And since it's also being repelled, it does this crazy zigzag which is not at all what I want. I just want it to spin. I tried changing the anchor point, which for some reason was at the center of the screen instead of the center of the object, but even that didn't help, because the anchor point doesn't move with the object as it is repelled. I could just manually keyframe the anchor point so it stays with the object, but that would be very tedious and I'm sure there's got to be a simpler way.

By the way, the spin behavior works fine if I apply it to an individual object; the problem arises when I apply it to a group.

Can you post a screen shot or zipped copy of the Motion project?