I have a different theory.

Originally Docker was going to be a container and CoreOS was a platform to run containers. They were both pieces in a traditional software stack.

Docker had a round of funding and the investors pushed Docker in a different direction: App store. Docker was no longer just a piece in the stack and CoreOS could see the writing on the wall: libOS. Docker would marginalize CoreOS the same way LXC was marginalized.

CoreOS is going on the offensive because they were tipped off on Docker's hand.

Occam's Razor.