It has to do with the sensitivity to shock, and the power of the shockwave. Primary explosives, particularly, and sensitive secondary explosives, are very sensitive to shock and so will propagate a shockwave even in a thin little tube, i.e. det cord. Some, like nitrogen triiodide, is so sensitive that little tiny grains will explode with a snap in open air, i.e. without any confinement whatsoever. However, less-sensitive explosives require confinement - but, which confinement can come from its own inertial mass. There is a minimum diameter of any given explosive that will propagate the shockwave down to the next length, and successfully set it off. Too small, and it goes out (and just scatters the remaining explosive without setting it off. In this warehouse, with 2,750 tons of the stuff, there are literally thousands of tons of material on top of the bottom layer. Is that enough confinement? Yes. Yes, it is.
Thanks...its own inertial mass makes sense