Stability Conundrum

For those that saw my launch of the 24mm cluster/rear ejection, on Saturday, you saw a couple of nice loop-de-loops! For the those that didn't, the rocket went about 100' to 200', then did a couple of loops before ejecting its parachute. The rocket did land safely with no damage. However it definitely looked 'unstable'. According to RockSim, it should have been stable. The calculated margin of stability is 1.64 - with both D12 motors inserted. I've attached a RockSim illustration. Any ideas?

I do need to modify the design as the fins were burned during the motor burn. It was a bad design to have the fins hang below the motor tubes. So replacing the fins with something with more surface area is not a problem, if that indeed is what is needed.

Thanks.

Is it possible the motor pods are disturbing the flow of air over the fins?

Thanks Guy & Boris. That level of aerodynamics is well above my understanding - which isn't much. Isn't any, in fact.

The rocket is a somewhat upscale of a US Rockets model - Dual 18mm Rear Ejection . The fins on that rocket extend directly out from the three tubes and is probably more stable than my arrangement. My plan right now is to replace the Big Bertha fins with simple trapezoid fins attached to the four seams of the three tubes. Hopefully that will do it. If I get my act together, it should be ready by the May 6th MMMSC launch.

Thanks again.