What are some reasons a 2.5 year old could have non obstructive sleep apnea?

Depends: Isually in a child that age it is due to enlarged tonsils and/or adenoids or less often foreign objects in both nostrils and they don't mouth breathe.

Central sleep apnea: In addition to obstructive sleep apnea, there is central sleep apnea, meaning something in the brain is mediating the apnea. This can usually be determined after having a sleep study done.

Tonsils & Adenoids: The most common cause of osa in children are chronically enlarged tonsils and adenoids. A competent ENT specialist can evaluate if this is the cause in a particular situation. They can distiquish if the condition is contributing to osa or may be csa (central sleep apnea).

Central vs T & A: Possible central apnea (nervous system) vs enlarges tonsils and adenoids. See pediatrician for possible sleep studies.