From what can be seen in the picture, I would expect it was an artillery round - possibly 105mm Howitzer, or something like a 250lb aerial bomb that penetrated the top front of the hull before detonating. It looks as though the top of the hull has been blown open from the inside, with the front part apparently missing, and the turret lifted off. The only thing else I can think of that could cause that kind of damage is the rounds store inside the turret detonating for some reason.