Heat buildup can cause this to happen. Qi is designed to monitor battery temperature, and will stop charging if the temperature gets too high, charging resumes once temperature drops again. This is done to preserve battery life, and prevent thermal runaway.
If your device is experiencing frequent overheating, it could be for a number of solvable reasons, mostly related to overloading the hardware. Pushing your GPU for too long is one of the quickest ways to overheat your phone. You can prevent overheating by not using your phone while charging and adjusting the car mount position under the dashboard's surface to minimize heat.
You can also try to lower the mount to an angle that the phone gets air from A/C.