Speculation: The AI tells it to walk away if it is standing and is near a blast zone, and this walking away has higher priority then any other action it can take. For instance, if you go to Bridge of Eldin and corner a cpu off to the side, you can just keep hitting it back as long as it stays on the ground and it will never do anything but walk back towards you, no matter what level it is. I'm assuming that the blast zone is a horizontal line under the stage, so when it's standing on it, it always thinks it's close to the blast zone, and should walk away. I could be wrong, but this seems a logical explanation. It would also explain why mastaklo's works, because he doesn't have the blast zone on the level of the grass.