I was told it's secondary nutrients and because of all the 'convenience food' (ie tinned meat and prepared biscuit meal) we give them, they're trying to get every last bit out of it. My dog used to do it, but I changed him onto the 'raw meaty bones' diet (gradually), so he only had fresh raw meat (scraps from the butchers, they give you them free because they only have to pay to have it disposed of) and marrow bones (no chicken or other small bones). He was 100% better for it (his skin and fur improved enormously) and he looked forward to every meal so much - and never ate his own poo again. And what could be more natural?
It didn't stop him trying to munch other poos if he came across themon a walk though, unfortunately.