I'm also having this problem, but it predates Mavericks, which I'm not running.
I'm using Mountain Lion, 10.8.5 on a 17" Macbook Pro (pre-Retina), and Word 14.3.9 (131030). And I concur that it happens with both Word and Excel.
I suspect Shelia Addison is on to something. My last startup was also after a non-graceful shutdown, where my battery had run out. Word and Excel were both running, and they may not have properly shut down.
What happens specifically is that Word (for example) catches the Option-H keystroke, and does indeed relinquish the active-app status, but its documents are not removed (hidden) from the screen.
I've just verified that quiting Word when it's in this funky state, and then restarting, restores the proper Hide behavior.