Loud explosion while backwashing black tank

Today I was doing my regular duty of emptying/backwashing the black water tank. I had emptied both the grey and black tanks and was in the process of backwashing the black tank. Now I know it says not to run the tank sprayer with the black tank shut, but I have always done this and watched the level of the water and shut it off to empty. Of course there has been the couple of occasions that I forgot the time and of course the water has always just gone up thru the toilet and made a mess inside. Thankfully it has always been after I have emptied and rinsed the tank out. But today, the tank wasn't quite full yet and all at once there was a loud boom (like explosion) and water started pouring out of the underbelly of the trailer (2002 3280 Montana 5th wheel). No water came out thru the toilet at all. It was getting late, so I wasn't able to take the underbelly covering off to inspect any of the tanks - will try to do that first of the week. I went on and made sure all the water quit running and made sure the water supply was off and left it alone. Does anyone have any ideas what happened and where or what I should start looking for first? Appreciate any and all help.