they die. It can be for a variety of reasons but never the same ones all the time. Adventurers live in a dangerious world and death stalks them froim the shadows.
They reach -10 hit points without stabalizing. Sometimes it's a dramatic moment, sometimes it'll be when a kobold gets one last lucky shot with a spear. When it comes to combat, you just never know.
When they run out of hit points. I don't fudge rolls, so they can die in really any circumstances. I've had people die in random encounters before, actually, although I try not to let that happen. (If it does, I try to come up with a plotline derived from the random encounter so the death has more plot signifigance.)