There are numerous warnings that, in time, AI will cause human extinction. A popular one is that robots will take over the world and eliminate all human life. Although that seems highly unlikely, the scenario that seems plausible is that autonomous weapons could eliminate humanity. If nuclear bombs are given their own decision making, and more and more military weapons are heading in that direction, then AI causing humanity to cease is not as far fetched as so many people believe.
Just Logical
It is very easy to understand that firing nuclear weapons is a decision that should be made without heightened human emotions getting in the way. As a result, removing the human element might seem perfectly logical in the future. However, no matter how many war games are simulated on computers, the end result of a decision made by a machine is why people are very concerned about the potential for AI to be catastrophic for humanity. See
autonomous weapon.