I believe that people who hoard are acting out of response to trauma, often related to a real of perceived loss. I've seen elderly people who remember (or whose parents spoke endlessly about) the Depression of the 1930's hoard food that they refused to throw away, instead buying extra refrigerators to store even more food. I've seen people who suffered the death of a parent hoard cars until their place looks like a junkyard, believing the longer they hang on to these vehicles, the more they'll increase in value. While this may be true of an original car kept in mint condition and garaged, it is not true of cars subjected to high heat, blowing dust, cold and years of sitting in the same place without adequate protection..
When my mother contacted me after 35 years (after illegally accessing my financial records and making ambitious plans for what money I had in the bank which wasn't nearly as much as she seemed to imagine), I began to buy and collect rare, beautiful and often expensive Geological specimens that I ran out of room to display. I also went beach combing in Southern Oregon quite regularly and amassed thousands of specimens. After I moved to Arizona, I began collecting specimens of metal, turquoise, rose quartz, petrified wood and possible meteorites... the message was only slightly cryptic: "Mommy Dearest, should you manage to outlive me, I hope you enjoy many happy hours sorting through this collection and attempting to determine what is and isn't valuable. Best of luck!" This woman abandoned me without remorse and I spent about 10k on therapy and medication to ease the panic attacks that began after I hung up the phone, having spent more than $100 to call her back and talk for the better (worse) part of four hours after she'd left a 4-minute message on the answering machine. People who knew me and saw just a portion of the minerals I'd collected thought I'd gone completely off the deep end. During the first and most expensive phase, I didn't quite understand my sudden obsessive "need" to acquire minerals. Yes, many are beautiful, but I'd gone over the edge! So there's a personal example of collecting beyond the point of anything sane. In my case, OCD had nothing to do with it. Heartbreak was the root of the problem.
I've met some strange people here in Arizona that I keep my distance from due in great part to the fact they are consumers of something euphemistically termed "bathtub speed". These people will go out into the desert and collect old rusted metal cans and attempt to turn them into "art" (unsuccessfully). Same people will enter the properties of others and steal outdoor water fixtures, door knobs, license plates, tools... they then meticulously mount these objects on to a painted piece of plywood or the sides of buildings and PAINT the objects, as well. I've even seen cactus that's been painted rainbow colors (which, unfortunately, kills whatever parts have been painted). And there's never enough room for them to store all these wonderful treasures, so they pile up. Ergo... substance abuse can be an answer to "what makes someone hoard."
I don't believe there's one blanket explanation. After awhile, people don't see the chaos piling up around them. When the reality of their situation kicks in, the chaos is often overwhelming and they give up before they begin. At least I've organized my massive "collection" into boxes that are stored in a large shed and labelled ONLY with the place of origin. Maybe I'll put up certain specimens and regional collections for sale..still undecided. I need the money a helluva lot more than Mom does! She gets a Social Security check... can't help but wonder which dead ex-husband's money she's collecting.