I am not an expert, but I've come to understand that bears are solitary hunters, like tigers. When you see bears together, it's because of salmon, usually. They have no problem foraging for salmon together, but prefer to attack (predation) alone. Now, if there's a group of bears that you can SEE, they don't see you as prey (they're already eating) or as a threat. What animal can threaten a group of bears?
There are primarilly two types of attacks:
1) The bear is hungry and wants to eat you. Mostly young black bears.
2) The bear feels threatened. Usually startled in thick cover, protecting its cubs, protecting its kills, or just a mad bear.
3) The bear is provoked. Usually involves tasty tourists who think bears are cute and follow the bears with cameras, get out of their cars to 'pet' them, etc.
Since bears aren't pack animals and the chances of a single bear attack are low, the chances for multiple-bear attacks are astronomical.