What is the cause of possible sleep apnea?

Depends. There are a lot of reasons for sleep apnea starting from birth allergies, to ill formed nasal passages, high vaulted palates, ill formed dental arches, nasal patency , enlarged tongue , high bmi to name a few and these are for obstructive apnea of corse it could be central and that is a different ball game.
See answer. Being overweight is a rsik factor, genetics plays a role, large tonsils and or uvula will increase risk for osa.