Aside from the obvious low battery power, there are two features that will trigger your device to turn off, Idle Mode and Incident Protection Mode. 

If neither is the reason why your unit is turning off, it is likely due to the card you are using. Format your SD card and give your device a full charge. it would also be good to check if your device's firmware is up-to-date to ensure it has the latest fixes and enhancements for optimum performance.