It could be a few things. Try starting it up in "Safe Mode" and see if the rebooting persists. If it does, then you will either have to try a full "Hard Reset" of the device, to fix it if it is a software issue. If that doesn't do it, then you may well have to have it looked at for hardware issues. Now if it doesn't reboot like that in "Safe Mode", then you likely have something on the device that is the cause. Bad app, virus, malware, etc. You would have to find and remove the problem, or do the "Hard Reset" to resolve it.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.