Mental health issues caused them to be homeless. People with severe Bipolar, borderline, or schizoeffective disorders can’t function in society without being heavily medicated, and a choice was made to not take that medication (I don’t blame them. Anitsychotics have super shitty side effects).
Sure, homelessness might make it worse, but their illness is what led to the homelessness.
My source is me. I already did the research myself, but you’re more than welcome to type a few words into Google if you don’t believe me.