In the context of someone accidentally spraying someone else, rodwha asked:
"How could such a thing occur?"
Bear spray comes with a safety pin or plastic safety clip, or perhaps both (mine had both). Once actually out in bear country, it's common to at least take the pin out (if present), but perhaps others take the safety clip off as well --- maybe they did a test spray and left it off (or dropped/lost it). Maybe they elected to leave it off to make it that much faster, easier to spray a bear at need; maybe so that they wouldn't forget the clip and get mauled while fiddling with it, something like that (?). That would be my guess, anyway.
Without the plastic safety clip in place it would be easy for the can to be bumped or dropped and start spraying, and with that concentration of bear mace, a little goes a long way. I kept mine strapped to my waist belt, and the can took some impacts on occasion when I put my pack down, or sat down quickly with the (forgotten) can strapped to my waist. If you sleep with the spray can near you, you could trigger it when fumbling around in the dark. Lots of opportunities for that sort of thing.
The plastic safety clip is designed to pop off quickly with thumb pressure, and so should be kept on anytime you don't actually intend to spray.