Hi Mike and LK, thanks for the response!
1.It happens from ca 15 knots wind.
2.My weight is 187 pounds. Yes, it feels like tail walking, loose control from ca 15 knots of wind. So you agree that the problem is the big fin? Why is such a big fin standard on a board like this do you think, and what kind of fin would you recommend in 25-30 knots?