It is probably due to an app, or multiple apps, that are running when you aren't using it. It could also be related to a virus or malware.
I would suggest, personally, turning off the tablet when you aren't using it. It is better for the device and battery life as well. You may also want to remove any apps that you really don't use, but are still running. And also make sure nothing is running when you do leave it on and aren't using it.
Lastly, run some good antivirus and antimalware scans on it.