A person who when seeing a new set of pronouns feels like they fit, and they use them. After a while they may do this quite a few more times, adding on to their list of pronouns. Soon their list of pronouns will become very, very long. They might feel forced to stop using some even if they like them, or not care and continue using them anyway. A pronoun hoarder does not have to use ALL the pronouns they hear about, but only the ones they like. They may use all the pronouns they hear about, however. It is not a very strict term.
Most of the pronouns they hoard are nounself pronouns and/or neopronouns, like scorp/scorps, xe/xem, and others, however they could use traditional pronouns as well.