The hysteretic behavior of Wells turbines is a well-recognized phenomenon. As it appears at nondimensional frequencies orders of magnitude lower than the ones studied in rapidly pitching airfoils and wings, the cause is likely to be different. Some authors found its origin in the interaction between secondary flow structures and trailing edge vortices. In this work, a detailed numerical analysis of the performance of a Wells turbine submitted to a sinusoidal bidirectional flow is presented. Computational results are compared with experimental data available from literature and suggest a new explanation of the phenomenon.