Unlike other Minecarts, TNT Minecart does not "die" in a fire, but instead ignites and getting ready to blow up. TNT Minecart is immune to fire damage before it blows up, but it still plays a breaking animation when on fire constantly, making it look like the TNT Minecart spins around 360° like crazy until it blows up. Other Minecarts break immediately 

Steps to reproduce:

1) Place a rail

2) Place a fire (should be on the same y level as a rail)

3) Place a single TNT Minecart

4) Carefully push a TNT Minecart in the fire

5) TNT Minecart ignites but continues to flip, spin and turn around very very fast until it blows up, it is even possible to see the bottom of the minecart.