Ted covered most of it. Also with a nose heavy model the up trim needed to balance the model will result in a very strong pitching up tendency with added speed either from power or diving. It will basically hate to be very far off it's best trim airspeed.

As it gets back closer to neutral the pitching up tendency from extra airspeed is greatly reduced.

In my experience any bad snap tendencies would be more due to the airfoil and planform of the wing along with the wingloading. I've flown a fair number of models that were very close to neutral stability and had no unwanted snap tendencies.

On the other hand an ARF Cub would snap and spin at the drop of a hat. It died on it's third or fourth flight. Good riddance.

The moral is that it doesn't always make sense but it does happen when you least expect it.