We're so sorry to hear about that! I personally have a few straps that I've worn nonstop that have started to warp and no longer fit the way they're supposed to. Unfortunately when this happens, it would just be normal wear and tear as the elastic fibers have simply broken down over time and no longer have the same elasticity as they used to.
If your strap hasn't been worn much and is new, this part is for you.
We typically find that new straps will warp for a couple of different reasons.
1. The strap was wet and stretches out or dries while moving around too much causing the elastic to dry in an odd way which in turn causes certain areas to lose elasticity while other areas do not causing the warped shape.
2. Certain chemicals coming into contact with the elastic such as bug spray or sun spray can cause the elastic to warp.
3. From time to time this will happen for no apparent reason due to defective elastic and that would fall under our warranty, please reach out to us with photos and we'd be happy to look into it for you.