Well I'm no expert but I guess the best answer is: It depends.
It depends on many things; how high it was dropped from, how much fuel is left inside, what it hits on the ground..... etc.
For example if you drop an empty tank into a soft field it will just probably bury itself in the dirt relativelty intact.
If you drop a full tank and it hits something hard(solid rock), and creates sparks when it hits, then mosty likely it will explode into flames like a 'bomb'.
So any number of things come into play when you drop a tank, and many different things happen to them.
Correct me if i'm wrong.