In reality this is a simple crew error of depth charges set to explode too early and the ship not moving away fast enough.
But the british admirality in SH3 never seems to tell their crews about this.
About the Bombs: it depends on the same factors as with torpedoes, when a big fat stuka-bomb penetrates the deck or strikes a main gun causing ammo to go boom.......well, not really healthy........remember bombs have a real high impact strength in adition to their explosion power. This is why here in germany so many unexploded bombs are found 10 meters underground. The impact alone already created a crater wich later was filled again.