I have talked to a number of homeless folks (I tend to go to a bagel shop for breakfasts most mornings) and a few of them are former engineers, both software and hardware. A little untreated mental unhealthy, a divorce, loosing the job, and boom. A lot of engineers have various rough bits to their ability to introspect and be flexible and so on. Being unwilling to bend in ways to go along to get along, and once you are older and you get off the track of normal life, home, etc., it's excruciatingly difficult to get back on.

For the matter, the one person I grew up with that has been homeless had a math, physics and econ triple major from Duke. Also no drugs involved, but maybe some untreated OCD.