The information in the tutorial above can give you more details about this and it's purpose to help.
When you have Windows Update set to install automatically, it only installs them at the time you have set in Windows Update settings. By default 3AM. If you have updates available to install until this scheduled time occurs, then this would be why you have this power button taken over like this. The power button will not go back to what you had it's action (ex: suspend, hibernate, etc...) set for until after these updates are installed either using the "Shut down" power button or through Windows Update. If you use the "Shut down" button, then you told it to install the updates and shut down the computer afterwards. This is why it doesn't ask for your permission first since you just gave it by clicking on "Shut down".