Therapy pressure is kept from leaking from the mouth accomplished by the seal made between the soft palate and the base of the tongue. Once xPAP pressure is applied that seal can be maintained as long as the mandible doesn't drop.
Once the seal has broken air can escape into the mouth where the lips will be unable to hold it back. You can test this out
1. Put on your mask, turn on the machine with mouth closed,
2. Place your tongue into the hard palate,
3. slowly allow the mandible to drop, your lips will part and no air should escape,
continue to slowly drop your mandible with mouth open until air escapes,
I can open my mouth about 3/4 to 1" before that seal is lost.
Note: If you had your tonsils removed as a kid they may have trimmed your palate and you will have difficulty maintaining that seal.
If you think you are mouth breathing you can try taping for a night or use a chin strap like the one SleepGuy offers called the Pap-Cap,
his is unique remains vertical so it doesn't pull the mandible back only prevents it from dropping. It can be adjusted where
there is no elastic pressure on the mandible yet it remains static to prevent the mandible from dropping.
someday science will catch up to what I'm saying...