I haven't heard of it before either, the default action is Ask and displays a toaster style popup notifying you of a Program update. But if you let it fade away or manually close it nothing happens. If however you click in the toaster popup it would proceed with the program update.
So I don't know if it is possible that you accidentally clicked on it or if the popup had focus and you happened to use the enter key that could possibly trigger it.
But it certainly isn't normal/usual for it to do this and even less so if you had it set to Manual before the update. Why had you set it to Manual ?