It sounds like malicious behavior (Human or virus) or a rule. To rule out it being a rule do the following:
Select the Deleted Items folder, and then go to View->Arrange By in the menu bar. Look under Custom.
If you have no luck after investigating all those options, maybe you’ll need to restore Outlook’s default configuration:
To restore Outlook views, toolbars, menus or settings, you need to reset the content of the OUTCMD.DAT file.
1. Exit Outlook;
2. Locate the OUTCMD.DAT file.
If you are running Windows XP and you installed Outlook on its default path, you can find the file in “C:\Documents and Settings\-USER-\Application Data\Microsoft\Outlook”, where “-USER-” if the name of your Windows login account. If not, search your local disk for the filename;
3. Rename the OUTCMD.DAT file to a different filename;
4. Restart Outlook.
Once restarted, Outlook should restore the default OUTCMD.DAT file from scratch and you should be able to use Outlook with its default view settings.