Yeah, they were reacting to the Kyuubi's chakra and lifeforce. But where does the Kyuubi get his chakra? seems to imply that it's gathered from nature, similar to Sage mode. Sorry, I was a bit tired when I posted that earlier, I accidentally left out that part. I also forgot, if the Mokuton is directly related to Natural Energy, that would explain why the Hashirama cells could suddenly go out of control when a person infused with them is weak or near death:, the cells are still gathering Natural energy, but with nothing to control it.