Basically it's just the way some tires wear. It can be minimized by keeping your tires properly inflated and like many others, I find 42 psi works best. Also some brands and models wear this way more than others. I have Michelin PR4's on mine and there is no noticeable cupping at all. The OEM tires, however, did it quite distinctively.
I don't think "riding hard" has much to do with it. I suspect it's just due to the way the tire flexes as it rotates that causes this wear pattern.