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Black bears are not herbivores. Black bears, like grizzly bears, are omnivores, although protein may form a larger proportion of a grizzly's diet than of a black bears.
Black bears eat vegetation, garbage, carrion, and, when they can get it, fresh meat. They are opportunistic feeders. Only polar bears are obligaate carnivores.

Grizzly bear attacks are more likely in areas heavily populated by grizzly bears, but, based on my reading, grizzlies rarely if ever eat people they kill. Most grizzly attacks are from sows protecting their cubs, bears startled by hikers, or male grizzlies mistaking heavily clothed large male humans for other male grizzlies in their territory.

Black bear attacks, based on numerous articles I have read, as well as science specials on educational programming on television, are of two types: females protecting their cubs - in which case the best defense is to play dead, and male black bears looking for food, particularly before hibernating, in which case active defense is your best chance for survival.

I don't believe bears "go nuts". I think there is a reason for everything they do - we just may not be able to read those reasons, and its always more difficult to predict individualized behavior in an intelligent animal. Compare snails with people for example. Snails always behaive like snails.

I stand corrected on the omnivore/herbivore point. I wasn't aware that black bears were similar to raccoons in that respect.

I deliberately put the term "go nuts" in quotes because it isn't exactly accurate -- as you said, many of these attacks have simply not been explained. I was always under the impression that an "unexplained" bear attack is more likely to involve a black bear than a grizzly bear for some reason. A Canadian female cross-country skier was killed in one such attack a couple of years ago while training in the summertime in Quebec, and as far as I know nobody has ever figured out exactly what that attack was all about.