I cannot give an authoritative answer but I read that the reasons change over time. For example when George Orwell (??) wrote Down and out in Pars and London the main reason was unemployment, as there was no social security. Then it became drink then drugs. I am not sure what it is now, but I would guess mental problems, possibly induced by unemployment or other stress.
Regardless of the reason we are all closer to being homeless than we want to realise. It takes only one bad debt at the wrong time or losing your job.
I read about one guy who had a thriving business. Then his marriage broke up. Being distraught he lost the plot in his business and lost everything. A few years later he died of cold sleeping in a shop doorway in Glasgow. The author of the book where I read that speculated that having to chase up late payers in forms where the accountants tried to delay payment in order to hang on to the interest prevented him paying attention to his marriage and had they paid on time he might be alive today, and maybe stil married.
But if someone tells me they are likely to become homeless and then tells me they voted Republican (USA) or Tory (UK) I tend to feel it is largely their own fault. Whether I say that depends on how close we are as friends: plus how violentthey are likely to get.