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Spherical tokamak plasmas are typically overdense and thus inaccessible toexternally-injected microwaves in the electron cyclotron range. Theelectrostatic electron Bernstein wave (EBW), however, provides a method toaccess the plasma core for heating and diagnostic purposes. Understanding thedetails of the coupling process to electromagnetic waves is thus important bothfor the interpretation of microwave diagnostic data and for assessing thefeasibility of EBW heating and current drive. While the coupling is reasonablywell-understood in the linear regime, nonlinear physics arising from high inputpower has not been previously quantified. To tackle this problem, we haveperformed one- and two-dimensional fully kinetic particle-in-cell simulationsof the two possible coupling mechanisms, namely X-B and O-X-B mode conversion.We find that the ion dynamics has a profound effect on the field structure inthe nonlinear regime, as high amplitude short-scale oscillations of thelongitudinal electric field are excited in the region below the high-densitycut-off prior to the arrival of the EBW. We identify this effect as theinstability of the X wave with respect to resonant scattering into an EBW and alower-hybrid wave. We calculate the instability rate analytically and find thisbasic theory to be in reasonable agreement with our simulation results.