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A hydraulic law every aspiring fire truck driver is taught: One pound of pressure will push a column of water (in a pipe) 2.3 feet high or to put it conversely, a column of water 2.3 feet high, exerts a pressure of 1 psi at it's base. With this knowledge, let's look at the problem posted i.e. the tank ruptured. Assuming a height of the tank's vent pipe opening being 10', the following applies: 10' divided by 2.3 equates to 4.348 psi at the top of the tank when the water exits the vent opening. Using a tank dimension of 24" X 48", the total square inches of the tank top is 1,152. Then if the 4.348 psi is multiplied times the square inches of the tank's top, we see that the total force applied is 5,008.896 pounds. At the point the tanks becomes full and the water is just at the point of entering the vent pipe, the tank pressure climbs from zero to 5,008 in just the short time it takes to fill the interior of the 1 1/2" vent pipe (maybe 5 seconds?). Lesson to be learned here is stay far, far away from over filling the tank. Another post by DQDICK suggested the possibility of a blocked vent pipe. Imagine if this scenario were to occur: the tank psi would be whatever the pressure of the source water supply. Thus 40 psi house pressure times the 1,152 square inches would equal 46,080 pounds of force on the tank top. If the object is to flush every surface of the tank's inside, perhaps a flushing wand inserted through the toilet would be a safer way. In my area, the house pressure is 80 psi. I would be uncomfortable running water for 5 minutes for fear I might overfill the tank. 3 minutes is my comfort limit when flushing, with an alarm set on my smart phone, and never l being further than reaching distance to the dump valve.
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