I disagree with this being due to tail heaviness. The angle of attack has to be whatever angle of attack generates the needed lift, regardless of CG. Flying faster will reduce the AOA needed to produce the nessesary lift. A higher wing angle of incidence will give the wing the nessesary angle of attack while keeping the fusilage aligned with the plane's direction of travel.
Reflexed ailerons could cause this by cambering the wing causing the zero-lift AOA to shift positive. You might experiment with moving both ailerons down slightly to shift the wing's zero-lift AOA to a negative direction, like flaps do.