Why do leather belts start out straight but curve after being worn?

When one bends over or even sits, the back belt loop pulls down on the belt where the pants would want to gap. Over time, this stretches it out and causes it to curve.

Your girth is bigger at the bottom of your belt than at the top of the belt. When you sit down your body becomes longer in the back and shorter in the front. Your pants will rise in the front while the back will pull down applying pressure to the back loop and your belt