Engagement rings were once "contracts" between man and wife. A sign to all other dudes that she was taken.
They'd agree to get married, and they'd buy the ring.
Contrary to hollywood, marriage was rarely about pure romance. It was much more of a business decision, (not completely, but more so than people would like to admit) at least in western society.
Then the diamond industry came up with the idea of the guy "surprising the girl" with the ring.
Guys shopping by themselves would spend twice as much, than if they went shopping with their wives to be.
The typical beta swallowed this new meme (you're a MAN if you're romantic, and surprise your girl with a ring) hook, line and sinker.
I would suspect that the growing trend of girls buying their own rings is a sign that girls don't want to feel obligated, even if they're married.
grrrrl power, and all that.
I suppose the takeaway from this is that if your girl starts talking about buying her own ring, because she's so independent, head for the fucking hills.