People who hoard usually have a combination of an anxiety disorder combined with some traumatic life event that has triggered this need. This typically involves a significant financial setback of some type. Persons with extreme hoarding and/or bizarre hoarding (think of some disgusting things that no one would "collect" and you get my drift) often have thought disorders as well. The deterioration in mental health makes the hoarding more prevalent and also odder.
Obsessive Compulsive Disorder (OCD) can range from the milder forms to more extreme and debilitating, such as the hoarder of weird artifacts. Someone who has an obsessive compulsive personality vs. disorder is not, themselves, affected by their disorder. Nope – rather they bug the heck out of you! Someone who has OCD is impacted by the disorder themselves and is distressed by their OCD behaviors. Someone with OC personality may only be annoyed that you are asking them to change their behavior, which to THEM is not a problem. Hoarders tend to fall in the personality disordered group. Most are not troubled by their "collecting" problem.
Compulsive hoarders have difficulty discarding items and what might be trash to someone else may be something that they think they might "need" later. As with other forms of OC, there is a component of perfectionism in many with this disorder.
Animal hoarders are their own breed (pun intended!). They seem to draw strength from the animals. Frequently interpersonal skills are poor and the animal hoarder feels more comfortable with their menagerie than with people.
It is my professional opinion that brain mapping would be very useful in further delineating brain functions affected by hoarding and its subtypes. Since decision making is one area that is a problem, it would seem that executive functions would be diminished. Thus, ADHDers should have a higher incidence of this disorder. Also, since executive functions are impacted first in many neurodegenerative disorders, including conditions such as Parkinson's, one may see some of this behavior displayed in the early stages of brain impact and this behavior would be atypical (rather than an increase in this type of behavior) for the individual.