Looks like people were complaining about the same thing back in '07, and that it happens fairly often with Alliant powder. That's not very comforting.
http://www.trapshooters.com/cfpages/thread_archive2.cfm?threadid=129116&Messages=19
http://www.trapshooters.com/cfpages/thread_archive2.cfm?threadid=129116&Messages=19