The horror that grows beyond all bounds of control, swallowing the world. What if the first atomic bomb would start a chain reaction that would just continue, igniting everything around it? The Earth would explode. The Sun. The Universe.
There are many thing that grows exponentially. Viruses. Bunnies. The economy. Computer capacity. But nothing grows forever. They will grow to fill the available space and consume the available food. After that, they will taper off. It's called the Sigmoid curve, or the S-curve.
The fear is that we will accidentally create something that will go viral. One such classical example is the machine that has the capacity to make more of itself. Even more so if we made the machine smart enough to improve it's functioning and defend against anything that would stop it from it's goal.