A Six has to be rolled for this occur, and the Dice Gods are usually off having tea with Crom and ignore your pleas for a huge parting explosion to fell those damn Orks who just killed your favorite tank with a power klaw.
The game we played yesterday, Orks vs Chaos Knights, there was a real lack of anything exploding big. Inspiration thus struck and we now have the Michael Bay 'Splosion Rule!
It is a house rule to be sure, it does ramp up the randomness and chaos on the field, so if that is not your thing please ignore it. But it does deliver more satisfying big booms.
If you fail to roll a Six on the Vehicle Explodes check, you MAY re-roll the die. Every time you re-roll you lose 1" off of the blast distance. If you wind up with a distance of Zero or negative, no explosion for you.
It is a fun addition if you feel like things just aren't blowing up enough. Clearly intended only for friendly games. Try the rules and let me know your thoughts on this.
Go Blow Up Some Stuff