Outlook 2011 Moves Old Messages to Top of Inbox

I run Outlook 2011 on two machines: one has Snow Leopard and the other has Mountain Lion. A few weeks ago, I noticed a change. When I would dig up an old message -- days, weeks, months, whatever -- and respond, after it was sent, Outlook moved the original message to the top of my inbox. It does the same thing with messages that I search for but don't respond to. All of this happens only on the Snow Leopard machine.

Anybody else experiencing this? Another longstanding oddity is this: http://forums.macrumors.com/showthread.php?t=1453262