I knew a few guys who had PhDs and were homeless, just kinda happens. One of my close friends worked the 2010 census and enumerated the homeless population in Atlantic City under the docks. From what he described at one point, there are loads of guys out there who are decorated veterans, former naval officers, PhDs, JDs, MBAs, and yeah living under docks.
Speaking from experience, it doesn't take much. When my father and grandfather both passed away within months of each other, I had to temporarily withdraw from medical school to take care of all kinds of arrangements and whatnot. Neither of them were financially in a good place, so when they died, any insurance they had went to paying their debt or medical bills. My dad's house had to be sold to pay his medical bills. He was an unemployed electrical engineer who used to work for Westinghouse elevators (which became Schindler) ... so he had no insurance and little savings left due to being out of work for a few years and living off his retirement while still looking for a new job ... but being in his early 60s there's no way anybody was going to hire him.
Anyway, I tried to get back to med school and was unable to secure loans from anywhere I tried. I had too much debt from undergrad and the first years of medical school (that's what a few loan specialists and bank directors told me at least). I didn't have any cosigners since I am an only child, have very few relatives who are still alive, and the few that are, have been laid off from work or even had to declare bankruptcy. One kinda distant relative tried to cosign for me but we were still rejected due to them not having strong enough credit. It probably made my situation worse since when I applied for loans after that, they saw all the rejections on my credit report, but I had to keep trying ... medicine was my dream at the time and I was doing very well so far.
Basically, all I'm saying is that it doesn't take much for somebody who's in a good place with their career to completely derail and end up on the streets (like I did for a bit over a year). Luckily I managed to eventually have some friends take me in, borrowed a few suits, went to more interviews, found a part-time job (which I lost about a year ago due to the economy and the small business having to downsize and lay me and two other people off), and eventually I met the girl of my dreams. I'm technically still homeless (because I'm "squatting" at her university housing until we move to grad school) but yeah, I'm hoping things will get much better since I'm making sure I'm more than qualified for graduate school, so with any luck, I'll be back in academia within the next year or two and things will begin to get better.