What does hoarding even fall under, anyway?
Since I've already made an armchair psychologist rant on the Kevin Allred thread earlier today, I guess I'll tackle this one too: hoarding, at least at the surface level, could be easily linked to attachment issues. Kids who don't have a parent figure that is constantly in their lives (either because Dad went to grab cigarettes five years ago or because Mom works multiple jobs daily) could bond with inanimate objects instead - either because it's the most noteworthy thing they have or because they found something valueless while they were feeling lonely, afraid, etc.
Every kid bonds with their toys or the places they like, even if they have perfectly normal, healthy childhoods. Young people bond very easily, it's a way for themselves to stay attached to their families and secure areas out in the wilds, or just stay safe in general. We still have a tendency to bond even when we grow older, which is why people can find love at an old age. Hell, people even have the potential to bond with their own kidnappers, terrorists and rapists (Stockholm Syndrome) which shows how bonding is an absolutely essential part of the human condition.
My guess is that, if you're deeply neurotic, it's easy to learn early on that habit of finding sanctuary in small little things and objects that strike your fancy. As you grow older and this becomes one of your primary ways of establishing a "safe space" for yourself, psychologically, that simply means you'll create a life-long habit of accumulating worthless junk and then never getting rid of it because you've bonded with your own personal Hoard, maybe not even the individual items but the whole thing. Getting rid of any item that gives you security is like getting rid of a loved one, in the mind of someone who has learned to bond emotionally with objects instead/as well as people.
Also let's keep in mind that as a natural instinct, in the early days of humanity, especially as hunter-gatherers, it would make a lot of sense to just go out exploring and bring back everything you find - fruit, smaller animals that you could eat later, particular stones or branches that could be used as crude tools, kindling of all sorts, etc. There would also be no reason to throw anything away unless you've run out of space or something's become rotten. So again, just another natural instinct of humans.
So just like people will bond with their childhood toys or people with a habit of making collections of particular things they find during daily routine (stamps, trading card games, sea shells, etc) it all boils down to basic human instinct, except for some people it gets out of hand, in the same way that for some people the natural, healthy human instinct for intimacy and procreation goes wrong and creates a serial kidnapper, rapist and killer instead of your perfectly average family man.
