We discuss the dynamic properties of a semidilute grafted polymer layer exposed to pure solvent. When the grafting energy of the head groups of the chains is finite, the chains desorb and are expelled from the layer. We combine Monte Carlo simulations using the bond fluctuation model to self-consistent mean field calculations and a scaling analysis. Chain desorption can be seen as a two step process. For strongly grafted polymers the limiting step is the desorption of the head group. The chain is then expelled by the osmotic pressure gradient. A chain cut off the wall is expelled at a constant velocity of its center of mass. The velocity decreases as the inverse of the molecular weight and increases with the grafting density. In the early stages of the expulsion the tension of the monomers close to the wall relaxes and the chain retracts. The retraction is independent of the molecular weight. Our most important result is that the desorption of the head group is a local process with a characteristic time independent of molecular weight. The desorption time increases exponentially with the grafting energy, it decreases as a power law of the grafting density. The exponent is close to 2 but the precise value is difficult to extract from the simulation.