If the tank was unvented and they were pumping it in from a bulk truck with a vane pump or gear pump it's hard telling what sort of pressure they could have built before it popped. 

I don't know for sure, but I've heard that if you are filling a five year old 300 gallon poly saddle tank with a simple single stage direct drive centrifugal pump you can blow it up if something (maybe a finger?) somehow plugs the vent hole.  I've heard that it makes a real loud boom when it splits, and the plastic whips with enough force to cause minor lacerations.  I don't know about any of this, it's just something I heard about.