Kalee20, you are bang on with your analysis. In fact it is easy to see from the recordings I made that the energy source for the high voltage spike is in fact stored magnetic field energy in the leakage inductance, not transformed energy from the other half of the primary.
Two things give the game away. Firstly the DC axis of the transient (even though the negative half is clamped out) is close to zero volts. If it was transformed from a voltage step it should be around 24V.
As can be seen also, the voltage on the drain of the recently non conducting fet rises slowly after that of course, because of the limited frequency response and hysteresis of the iron cored transformer ! (It might be a different case if it was ferrite).
So clearly, the transient is powered by stored magnetic energy in the leakage inductance, exactly as you said
