From what I know, it is a trend. Yes, mostly a black trend, but to lean over is the norm for a lot of young black men. When you are leaning, you are seen as ‘laid-back’ and ‘relaxed.’ It’s just one of those things that a lot of young men do. Why do some white teens color their hair purple, or pierce their eyebrows? Same reason: to follow a trend.
Why are they bad drivers? I don’t know. From what I observe, a lot of young people are bad drivers. Not enough experience, I imagine.