There are a lot of reasons you may have noticed your dab rig cracked. Heating a dab nail usually involves a torch, and while the dab nail is designed to handle super-hot temperatures, your dab rig is not. Subjecting a part of the dab rig to direct heat can cause the glass to crack since quickly heating or cooling glass can cause it to break or shatter. Rough handling of your dab rig can also cause it to crack. If this was the case, consider a silicone dab rig.
.