JMO, maybe they both went out there alive, something happened, and instead of leaving her there, or calling 911, or driving to an emergency room, he took her home. Panic, or no cell service, or not wanting to be implicated, or whatever irrational crisis reaction. He put her in the frunk because he couldn't face driving sitting next to a body, or he was afraid someone would see it on the way. Maybe he told himself he'd deal with it back in town, or in the morning. But he put it off, and it wouldn't take very long for the problem to be seemingly insurmountable. An accident (possible, benefit of the doubt here) immediately reported is one thing, a decomposing body that you removed from the scene is quite another. JMO.