I always thought it took muscles to keep your mouth closed.
This article says when a person is about to die the body relaxes, facial muscles relax and the jaw can drop. (Mouth open ) after death, once the brain can no longer send signals to muscles or no saliva to unconsciously swallow. It will remain in that position after death (the facial muscles are the first to freeze can be 20 min into rigor)
Basically it's saying the mouth opens shortly before death/ and before rigor sets in and will remain that way if not closed before it locks.
article about stages of rigor.
Rigor mortis is the third stage after death, in which the muscles harden and become stiff. Learn about its causes, stages, and more.
www.medicinenet.com
If anyone has been with a person dying they might remember having to swab someone's lips with a little sponge.
Was there only one video that they said her mouth was open or 2? ( which would show time) I know they said her body was in the same position, (mouth open?) I'm assuming thats why they think she was already deceased in those early morning videos.