One of the bears fed on the victim, that's an automatic death sentence. The cubs learn everything from momma, they're concerned about learned behavior. Here's a link to the latest article in todays paper:

http://www.greatfallstribune.com/article/20100731/NEWS01/7310309/Grizzly-euthanized-tests-link-her-to-maulings

There are no theories on what provoked the attack, like someone said, they're bears, who knows?