Hoarding is a recognised mental health condition often triggered by trauma.
It's the tip of the iceberg.
Underneath may be so much more:
Neurodivergence.
Bereavement and grief - sometimes even from the death of a pet.
Abuse survivor.
Other mental illnesses.
Relationship breakup.
Loss - I remember a woman exasperated by her hoarder mother. Felt like her mother was doing it deliberately to be controlling etc. Learned her mother had escaped with her own parents as a refugee with only a suitcase each - and these had been stolen on the way.
Chaotic family history where they'd never learned how to organise, clean and tidy because their parents couldn't or wouldn't themselves.
Guilt - anxiety about waste and sustainability, 'it might come in useful some day.'
Things just getting out of hand bit by bit - collections, collections of collections, things that might come in useful.
Sadness, loneliness, depression - the stuff can feel like security, familiarity, reassurance. If there's enough of it, it can become like a den. A fortification against outside dangers. Collecting it feels nice, comforting.
Shame. Once it gets out of hand it can spiral, becoming all too much and the hoarder has lost control but doesn't want anyone to know so feels they can't ask for help. And it spirals some more.