We will present simulation results of the flow dynamics in the co-orbital region of a protoplanet, with an emphasis on the evolution of its potential vorticity profile. We show that the flow's p.v. profile develops inflexion points due to the presence of shocks. The continuing change of the p.v. profile eventually leads to a secondary instability, the nonlinear outcome of which is the production of vortices. Born in the co-orbital region, these vortices undergo close-encounters with the planet, exerting strong torques on the planet which show large fluctuations in the magnitude and direction. Implication for the migration will be discussed.