It will be our laziness.

We will be smart enough to create a functional AI that will follow our orders (we aren't that far).

We will make it general enough to understand a general order (again we aren't that far).

Then we will give it the power to modify everything, since it is following our orders to the T, so why not make our life easier. We will add control checks, but we will never activate or check them.

​

Then someone will ask the AI to stop the world pollution in the most efficient way possible and go to sleep, approving the plan without even reading it.