Bils AT system... same folks that gave us the Carl Gustav RR that the Rangers use. The corkscrew effect is normal right after a ATGM launch. No matter how much you train the gunner, there will be a couple of seconds the back-blast of the launch will wipe obscure the sight picture. After the launch engine burns out... the main engine burn starts and the steering motors fire to begin tracking on guidance signal from the guidance module. Its a good weapon because it uses a top down attack to hit the thinest part of the armor on a tank. What you saw at impact was the HEAT detonation with probably a DU penetrator. The penetration of the armor creates the plasma jet that sets off the ammo in the tank.
US M1 series tanks have the ready ammo stored behind blast doors in the back of turret. Soviet bloc made tanks have the ready ammo in racks around the turret. The Leopard that was used for the demonstration did not have a full load of ammo. To say that a hit like that on a war loaded can be dramatic is an understatement. The ammo will often go off in one, major catastrophic explosion.... taking the turret off.