Try starting up the tablet in "Safe Mode" and see if it it happens in there. If it doesn't, then you likely have something fishy on the device. Bad app, malware, virus, etc., that needs to be removed. Now if it does happen in "Safe Mode" as well, and it is your own tablet, I would contact the manufacturer. If it isn't your own, and day it belongs to a parent or an employer, then you need to talk to them about the access.
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.