If your belt is bunching up in the back, it is likely because the belt is too big for you. When a belt is too big, it will naturally bunch up in the back because there is too much fabric and not enough structure to keep it in place. The best way to fix this problem is to find a belt that fits you better. You can either purchase a new belt or have your current belt adjusted to fit you better. If you have a belt that you really like but it is too big, you can also try wearing it in a different way. For example, you can try wearing the belt lower on your hips or even backwards. By wearing the belt in a different way, you can often make it fit better and prevent it from bunching up in the back.