Pit of pestilence and disease?

[Mossdog]

Mossdog, that looks pretty boggy. Could it be that they all became trapped in the bog and couldn't get out - i.e. no influence of humans?

Could be. But it looks as they have been piling up there over quite a long time span, not a one-off event. As it's adjacent to a well worn quad bike track, I'd expect it to have been noticed as a danger point for sheep and fenced off.