It looks to me like the first grenade exploded in the barrel with the breach block closed. The explosion may have forced it open, or the crew inside may have opened it to find out what the heck just happened. The second grenade let them know.
Look at the way the tank rocked after the first grenade.
There was a live round in the barrel.
The round was fired with the grenade in the barrel or the round was touched off by the grenade.