Sorry for your loss. It looks like a mass extinction event with survivors.
May I ask how long were you away? Does your tank cover have any holes for air to pass through?
According to
this, aro is able to draw oxygen from the air. The article does not mention for how long, but an intelligent guess would be not too long...
How long could an aro or fish survives in oxygen depleted water with waste/decaying dead fishes/ammonia/nitrates built-up? The death is probably slow and painful.