Mine has done this a few times, too. It does seem like it has happened when I've had multiple alerts although I also have a lot installed (including SPB Mobile Shell) so it could be just about anything. I have had to do a soft reset to get it to stop.
It's mildly annoying, a little funny, but as a former Treo owner, the number of soft resets I've had to do is still minimal compared to daily (or multiple times a day) with the Treo.