The property DamageType in the weapon's Weapons.con file specifies the type of explosion. For explosive packs (and almost every other explosive) this entry is:
which means that the object explodes on impact. Landmines and flak guns are the only weapons with a DamageType of 4, which was introduced with the v1.4 patch. This setting is used to allow the projectile to explode when it is near another object. The property ExplodeNearEnemyDistance sets this explosion distance.