Well, there are times when you have to wear a cap backwards for it to stay on your head. Like in your bass boat, four wheeler, motorcycle, fast horse and etc.
That said, I have this theory that when you see some kid in a car, with his cap on backwards, he's likely in a methadone rehab program.
I think the fad started with kids imitating catchers. The catchers turned their caps backward so they could wear their masks.
Catchers like Yogi Berra were cool and had that rough and ready look about them. It was cool for a while but now everybody does it.