[Imotepchief]

The explosions trigger from flame crits, which can only occur if you do more than 50 fire damage. Soldier skills that can gain the blazing trait, either through nerhaven or a mars badge, can trigger the effect, at least in the case of crescent rook and bright bishop from black knight.

There is a also the volcanic enchantment, which seems to reliably cause the explosions to trigger if you have high enough fire damage + high power weapon.