My theory behind this, which the article doesn't touch on, is that maybe alternative lifestyle parents are far more likely to go for unusual names (I know a few "hippy" parents myself, and their kids are more likely to have less common names, "River" or "India" being two I know off the top of my head). These same people are less likely to push the child in traditional directions - study hard, work hard, good job, head down. They are far more likely to let their child grow up in a less pressured way.

Anyway, that's just my personal observation from hanging out with friends with less conventional lifestyles over the years.

(My other theory would be that the kids raised in less conventional ways, would be more likely to follow their passions, and enjoy their work, rather than becoming a corporate lawyer or accountant. Probably have less mental breakdowns or mid-life crises, but this is all speculation on my part).