This is odd behavior for the New Outlook 2011.

So we can attempt to figure out what is causing this, can you supply some additional details.

1. What exchange platform are you on? You can locate this information if you have access to Outlook Web Access.
2. Can you create a rule to cause a work around? this will be temporary until a solution is uncovered.

Note: The following link is a better mac forum:
http://www.officeformac.com/ProductForums/Outlook/