Many ways to end up there. Falling chronically ill is an easy to understand one, even depression can leave you completely unable to get or keep a half-decent job, but many of the better-paying jobs also require some amount of physical fitness. And if you live in the US, any kind of emergency can single-handedly wipe out all of your savings.

Also spending some time in prison might do it, which often happens for bullshit charges. Or just plain old generational economic deprivation - it's a feedback loop!