It does not require a High Explosive.
http://www.sinooan.com/do-you-know-much-of-disaster_91.html
http://www.hysafe.org/science/eAcademy/docs/3rdfesshs/presentations/ESSHS2008DorofeevSB.pd
Texas City 1947 is an example of this phenomena.
Prayers to the survivors and those that have lost loved ones.
Correct. Also I read in an early report that there was a fire, they used water and then the explosion. Water and AN is endothermic but after a certain point it becomes exothermic so that could have been a factor.