An article in the Alaska Dispatch has some new details.

Apparently the group was hiking down the winding creek, not crossing it, when the attack occurred. It was also raining heavily which it's thought, along with the creek itself, could have masked the group's noise.

Based on this and the geography, it's thought the bear didn't realise there was a large group:

"As the students, Stuemke and Ford have described the creek bed in which everyone met, it is quite probable the bear at first thought it was confronting only Berg.
...
"She (the bear) probably didn't realize the others were there," Stuemke said. Had she known the size of the pack she was confronting, bear biologists believe, she likely would have smacked her cub to get it moving and chased after it away from the area. Instead, however, she attacked Berg, cracking his skull in the process, to neutralize what she apparently perceived to be a solitary threat."

Also, it looks like no one in the group was able to deploy their bear spray in time:

"[Berg] was attacked and knocked to the ground before he could grab the bear spray he and others were carrying with which to defend themselves.
...
Some of them were trying to get to their bear spray. They never did. There wasn't enough time."

The big takeaway I see from this is to ensure you can always get your bear spray (or firearm) out and ready to fire in seconds (or less if possible). You don't have much time to react.

This article also has more details about the first aid administered by the group following the attack; it sounds like they did an amazing job.