Why would a red ear slider bury itself in an aquarium?

Maybe it is too cold, and it is trying to keep warm. Does the instruction manual say to provide a separate heat lamp at one end to raise the temperature significantly? I bet it does. If you have such a lamp, you could try putting a red bulb in it so it is warmer radiation than a white one. Maybe you have one, but the tank is too tall for the heat to reach the bottom of the tank. Check online to see what the ideal temperature should be in that tank and get a thermometer and measure it.
Maybe it is laying eggs?
Phil