There is no defined causes yet. Science believes there may be a link to one's genetics. For example you may have a higher chance if your mother was a hoarder.
Hoarding can be caused by a fear of deprivation. It is considered a type of obsessive compulsive disorder and occurs in all age groups from childhood to late adulthood.
What causes hoarding syndrome is rooted in an issue called Obsessive Compulsive Disorder (OCD). This is a psychological condition (the cause) in that one has an inability to dispose of items that they have no need for.