I am thinking there could be an explanation: Couple starts out on the easier Maze Loop trail. They complete the hike, then decide to attempt Smith Water Canyon on the same day. As they are not in trouble yet, crossing the main road does not faze them. As some point on the hike up the canyon, they get overwhelmed. Or, they get confused and hike into a canyon that they think is Smith Water canyon. They then get disoriented? I think that is a good use of Occam's sharp razor. Another experienced hiker commenting on a Grand Canyon case also stated that National Parks are well patrolled and really don't have resident squatter colonies of the aggressively drunk / drugged nor the commercial pot growers that some National Forests or BLM lands do have.