I noticed it happen a couple of times with some of the Bell guardians who got summoned whenever I try to go through Belfry Luna. Personal thoughts on it are the guy who got summoned did so by accident, didn't actually want to stick to it, and so just jumped off the ledge because dying as a grey phantom incurs no penalties whatsoever.Going by half the bloodstains I see, most of the deaths by falling are preceded by the person stopping atop a cliff for a few seconds, contemplating their lives and launching themself off the edge for no clear reason, so I'm not really sure.
It's amazing how many times I too have seen this. People want to commit suicide but not in the real world. Dark Souls allows them that choice.
Heard it happens alot with the Rat covenant as well; players essentially killing themselves because they can't be fucked going through a retarded maze.