The only way you would have any kind of auto backup would be if you have the tablet set to sync with say a cloud account, a google account, or something similar. You would need to turn off that option to move the videos to the card.
If it still won't let you move them, then you can try connecting the tablet to a computer, via USB cord, then move the videos (and any other data) to a folder on the computer. Once moved off the tablet, then you can copy them over to the SD card (while it is still in the tablet). I say move to the computer, and only copy to the SD card, so that you will have a copy saved on the computer.
Better to back it up now, than to lose it all later.