Hello JCA.
Though many have answered the question already, as a practicing clinical psychologist, I’d like to give it a shot.
I thoroughly disagree with those who diagnose this condition as OCD. It may have similar characteristics in a small percent of the population but the majority of horders are people with ADHD.
They cannot make a decision about what to keep, where to put it and whether or not to throw it away. Since sorting (leading to disposing of) is a rather dull task, the mind wanders and suddenly it seizes upon something interesting either in the heap that’s being sorted and thrown out or somewhere else in the external environment. The throwing out stops. Time stops. All energy is then directed toward the new more interesting task.
It is most certainly genetic. The upside of ADHD is that they are enormously creative people and when they find their nitch they can be hugely successful. I must say, its hard on a marriage. If they choose a partner who is orderly and can throw things away, it leads to much conflict. If they choose another “creative” with ADHD, their environment is always a mess unless they can put external systems in place such as cleaning help and professional organizers.