It sounds like your tank underwent an acute gas supersaturation event. Gas supersaturation can occur any time the pressure of dissolved gases in water becomes higher than atmospheric pressure.
When fish breathe this supersaturated water, the dissolved gases may leave the bloodstream and form air emboli (bubbles): gas bubble disease. These emboli occur in a variety of tissues, ranging from the eye to gills, along with internal organs such as the heart and swim bladder. In acute supersaturation, you may also see emboli in the gill lamellae, opercula and around the mouth. Additionally, ‘silver lines’ (which are actually long gas bubbles) may be visible between the fin rays.
The clinical signs of gas bubble disease include abnormal buoyancy (which you’ve noticed already), caused by excessive gas in the swim bladder; abnormal protrusion of the eyes, or exophthalmia (the swollen eyes you mention), and often sudden death (your massive die-off), due to an embolus in the animal’s heart.