How do TAB & SHIFT-TAB get reversed in Entourage?

Question

In Microsoft Entourage, I can use Tab to move the highlighted selection from "Inbox" in the left-hand column:

...to the list of email in my inbox:

And Shift-Tab returns the selection to the left-hand column again.

But once-in-a-while when I return to Entourage (but never immediately after launching it), Tab and Shift-Tab are reversed. That is, I need to use Shift-Tab to move the highlighted selection to the list of email and Tab to return the selection to the left-hand column.

I don't know what causes this, and I find it so annoying that I'll restart Entourage just to restore Tab and Shift-Tab to what my muscle-memory expects.

What is causing them to get reversed, and can I un-reverse them without restarting Entourage?

Update: It just happened again, and I confirmed that Tab and Shift-Tab are reversed on both my external keyboard and the built-in MacBook Pro keyboard. So I'm pretty sure it's not "stuck keys". I'm guessing it's either a bug in Entourage (in which case I'll continue to restart when this happens) or there's some way to change the tab order, which I'm accidentally activating. If it's the latter, maybe I can learn how to avoid doing so, or learn how to switch the tab order back to the default.

Update #2 (responding to JRobert's comment): I just went to Entourage and found the keys reversed. They're not reversed in Safari (tabbing between the URL field and the search field), and they're not reversed when I use Command-Tab and Shift-Command-Tab. But they're still reversed when I return to Entourage. So my guess in update #1 is not disproved.