It isn't AI specifically. Any intelligence capable of taking over or shutting down a wide range of systems, or even just a few critical ones, needs to be closely monitored if there is more than a small chance of it causing massive harm.

Now, if that intelligence also far exceeds our own in certain aspects but is not empathetic enough, and if it were to have a keen understanding of how to control humans, and if it were much more able to quickly control and adapt software and hardware, then it would become a much bigger risk.

But, you're right- in theory, there could be a natural intelligence that could be the same level of risk as an artificial one. Most of us, including myself, are just not aware of one.