Bathroom flood - What did I do wrong?

I would check the black tank flush system. Mine goes from the hose inlet, up in the back of the vanity and then back down into the black tank. There is a vacuum breaker at the top of the loop. (this is nothing to do with the drain system from the vanity bowl.)

One of the fittings was loose and when I turning on the black flush, had water spraying up inside and running out through the floor openings behind the water panel

Easy to check - take all the drawers out and have someone watch as you turn the black flush on.