^^^ It is most likely because the jeans are too big. In order to keep them up, he is wearing a belt. That cinching causes all that fabric to bunch up in the crotch area. You can also tell that his jeans are too big for him by looking at the front belt loops. See how they are pulling? It's because the belt is tight and the denim is not; therefore, the fabric around the waist is also bunching up.