The report says that they ran ~173 yds (520 ft) before contact. (ref summary on page 2 of the pdf)
The bear alerting at 100 yds and starting to run toward them doesn't surprise me. The chase seems a little long, but the tracks indicate that the bear may have turned and stopped (pdf pg 6, "The bear's reaction and know history")
While the bear appears to have started chasing before they started running, the running (and yelling) might have encouraged the bear to continue the chase and attack. (Dropping to the ground and playing dead is the generally recommended defense in such a situation.)
I don't see any reason to suggest that the attack was predatory, but I still think it is consistent with a defensive attack. (The bears also appear to have left the scene just after the attacks.)
This report is just the investigatory report--I couldn't find a conclusion. However, a bit of Googling turned this up:
http://blogs.laweekly.com/informer/2011/07/yellowstone_bear_attack.php
Doug