The pointed nosecone on the top of the rocket came off and it lost aerodynamic stability, and started to tumble. The tumbling caused the fuel and oxydizer to sl

The pointed nosecone on the top of the rocket came off and it lost aerodynamic stability, and started to tumble. The tumbling caused the fuel and oxydizer to slosh around the very thin (and lightweight) tanks that are not designed with withstand that kind of force. The sloshing accelerated the tumbling. The thin tanks rupture causing the fuel and oxidizer to mix in the vicinity of a lit rocket engine. BOOM.