In general, there are 2 levels of crankcase explosion: primary & secondary crankcase explosion.
The primary explosion is caused by internal atmosphere of the crankcase. Secondary explosion, which is caused by fresh air entering into crankcase through any means such as unsettled relief doors, is more violent. The primary explosion is released through relief doors but secondary explosion is extremely severe and in some cases, blew away the crankcase doors, causing subsequent fire in engine room.