Generally it would not be included in the definition of trauma. As a rule, trauma is defined as, wounds that result from sudden physical injury or violence.
Its all specualtion. My main theories are hemorrhagic brain injury, resulting from a pre-existing vascular disease or a seizure that, becuase of the harness, positioned him in a way that he couldnt maintain his own airway. Statisticaly the majority of pediatric cardiac arrests are secondary to resperitory arrest, I am partial to the theory of a hemorrhagic brain injury.