I tend to lump browns and grizzlies together even though there are some minor differences with browns (kodiaks) having evolved to a larger size due to their fatty fish diets. Blacks are smaller than either (although some blacks in Alaska can also get very large due to rich diets).

One thing I've read is that even though grizzlies can be more lethal than blacks because of their greater strength and size, they tend to maul their victims but not necessarily kill them outright. Most people are actually survivors of attacks. Blacks have a bit more tendancy than grizzlies to stalk a person and they tend to finish the attack to the end rather than maul and leave as the grizzly would. These are tendancies, not written-in-stone behaviours, mind you. There are two three situations where bears are most dangerous -- when a sow with cubs is involved or if you come between the bear and it's food or if your dog gets aggressive with it. Otherwise they will most likely leave the area to avoid an encounter or they will huff and bluff charge in an effort to get you to move.

When I saw the follow-up articles about the first one or two people encountering the bear AFTER crossing the stream, it makes a far more sense. The bears may have not have known there were more people or a group which explains the idea that there were 7 hikers. And as others pointed out, the sounds of the stream and air currents could have masked the sounds and even the smell of the hikers as they approached which lends to the idea that the encounter was a surprise to the grizzlies. The first two individuals may not have had one of the 3 bear spray cannisters.

In any case, I wish the hikers a speedy recovery and all the best going forward.