Black Water Accident

If I am reading your story correctly, I think you answered your own question. You state that you opened and closed your valves to lubricate them prior to your trip. In doing so, any remaining fluids/chunks from your last dump, plus the antifreeze that you had put in your tank to winterize, would have poured down the tube and filled up the space between your valve and your outlet cover. When you closed the valve you sealed that space, trapping the fluid between the valve and the cover. When you removed the cover, that pressure and fluid would have been released the - hence the unpleasant shower.

We've all been there in one way or another! Happy trails!