I'm no engineer, and when By Smalley (rocket engineer and cartridge designer) tried to explain Secondary Explosion Effect to me, I simply got lost in the explanation. I'm certain that I can't even begin to reproduce his words, but the SEE phenomenon has something to do with reflected wave fronts, harmonic amplification and localized powder heating. I think. The true SEE requires slow powders coupled with a certain amount of airspace and also depends on case shape, IIRC.
Other "blowups" can happen, but are not caused by a true SEE, the cause usually being double or excessive powder charge. The classic blowup is the .38 Special target load of 2.7 Bullseye under a deep-seated wadcutter, except that the burst load probably contained a 5.4 Bullseye double charge.
