SEE = Secondary Explosive Effect (aka - Detonation).

If I understand the basic theory correctly, there is a partial burn of the powder charge, the projectile moves forward and hesitates or hangs up, a pressure wave and intense heat occur, the remainder of the powder charge explodes creating amazing rifle shattering pressures.

I believe the most accepted culprit loads are under-loads of slow burning propellants.

(FWIW - The UK 6.5 X 55mm Norwegian Krag rifle mishap involved a load using 140 grain bullet and 34.4 grains Accurate 2520. I don't think Accurate 2520 is a slow burning propellant and 34.4 grains is a bit above the 'starting load' in some tables).