The likeliest possibility is that, while he wasn't breathing, he wasn't dead yet, and could have recovered if he'd been given proper medical care instead of being buried alive to preserve the officer's reputation.
The accident's not the issue here: They didn't even try to resuscitate him (which is possible, even when someone's not breathing and their heart's stopped) or administer any first aid - they just buried him and he's considered a missing person who never came back from leave, probably court-martialed in absentia when he didn't return at the end of his leave. All to make sure the unit didn't have to report an accident. Anyone would be mad as fuck about that.