When the flushing the black tank using the hose connection, and a closed drain valve, the tank will fill to capacity, then start pressurizing the tank, while trying to force water out of the vent tube. Essentially you are pushing against 35-45 gallons of black water and forcing it up a vertical tube about 2 inches in diameter.
The tank bulges, and may pop a band or one of the bolts holding the bands. If you opened the toilet valve, it would surge that way. Larger hole, and lower than the opening of the vent tube.
When you open the black drain valve, all that pressure relieves, and jets out of the valve.
When flushing, I NEVER walk away from the project. Stay right there, and work the valve or count to 100, but don't walk away to do anything else.