Twisted limbs. On longbows, it's most often caused by improper stringing, i.e., the step through method. Sometimes, if the bow is left standing on one tip for months or years on end, a limb may also go off a little. And sometimes, the limbs aren't made straight when the bow is built. That's a possibility. You just may never have noticed before. It's always one of the first things I look at when I buy a new/used bow. If it's not off too bad, can just continue to shoot it. Applying moderate heat, with a hair dryer, for example, and twisting the limbs in the opposite direction of the current bend/twist may straighten them. Good luck.