It's the sort of thing that can even happen almost accidentally, if you're prone to it. A lot of people have a particular kind of drug they're vulnerable to.
But not everyone has a lot of self-awareness, or familial awareness. So, some people start like that, especially in the middle class and above. People can follow a similar course with uppers, starting with something like Ritalin by prescription.
But there has also been a massive push of smokable heroin in the suburbs, which overcomes the low-class stigma about injecting. For some reason some people think it's somehow "better" or less dangerous to smoke it. I actually know some kids from my high school who wound up on heroin that way. It was a pretty well-to-do suburb -- not the kind of place you would guess would have a massive heroin problem. Just kids taking silly risks, thinking that somehow the method of use makes a drug less addictive.
And then, there's also the stereotypical situation: people who are really down and out going to harder and harder drugs to block out an unbearable reality. The predicting factors to this are basically what you'd expect: history of abuse, homelessness, mental illness, etc. Basically people who just don't see a point to their lives, and thus don't care about preserving it. Gay teens who are thrown on the street. Young women who are raped at home. Untreated traumatized vets. Untreated depressives, schizophrenics, and bipolar people. All those sorts of things, which are still unsettlingly common.
