[Honclfibr]

As others have said it's probably a bad contact either inside the horn switch or from the horn to ground. Bad contacts can have a diode effect that avalanches once your stator is generating sufficient voltage.

I'd suggest selling the katana and buying a dedicated trackbike. Katanas aren't the best track weapons in terms of motor and suspension, and the money you can fetch now that riding season if upon us should buy a former racebike which will make a much better trackbike.