If the tank is fitted right at the back,at an accident it will come off its bolting points and travel to the front picking some speed and hit you.That's dangerous.
If the tank is fitted right behind you,and there is a bulkhead or a metal bar,it won't pick any speed so no impact,it might just push a bit.
If we look at those crush tests,there is a gap between the driver seat and the tank.
And they should test it with a bulkhead or a metal bar.
That's just my thought.