Just a few comments:
- Do you mean that you are looking at the tower-base fore-aft moment, i.e., moment driven by fore-aft forces?
- Is your simulation with steady wind and in steady state? Otherwise, the solution is likely oscillatory and effect by inertia terms.
- How is your gravity-induced moment calculated and does it include the effect of tower and blade deflection?
- Are you considering direct aerodynamic loading on the tower, which will also contribute to the moment?