It's normal to sometimes wake up to find you've removed the mask in your sleep. If you move a lot in your sleep, you may find that a full face mask will stay on your face better.
You may be pulling off the mask because your nose is congested. If so, ensuring a good mask fit and adding a CPAP-heated humidifier may help. A chin strap also may help keep the device on your face.
If this is a consistent problem, consider setting an alarm for sometime in the night, to check whether the device is still on. You could progressively set the alarm for later in the night if you find you're keeping the device on longer.