That is strange indeed. Are you certain that it is only by pressing the shift key and before pressing any other key that pasting is occurring? If so, I'm tempted to wonder whether it is a Windows glitch that has been created, perhaps even an accessibility in which case a repair Windows cycle might be helpful. I might also suggest opening the All commands list in Word's keyboard shortcuts (Options/Customize Ribbon/Keyboard shortcuts Customize), and scroll through the various Paste commands to see if a shortcut has been created. Still, Shift is not, to my knowledge, a sufficient command for paste.