In light of the race towards macroscale superlubricity of graphitic contacts, the effect of grain boundaries on their frictional properties becomes of central importance. Here, we elucidate the unique frictional mechanisms characterizing topological defects along typical graphene grain boundaries that can vary from being nearly flat to highly corrugated, depending on the boundary misfit angle. We find that frictional energy dissipation over grain boundaries can originate from variations of compressibility along the surface, heat produced during defect (un)buckling events, and elastic energy storage in irreversible buckling processes. These may lead to atypical nonmonotonic dependence of the average friction on the normal load. The knowledge gained in the present study is an important step towards the understanding of frictional effects of extended grain boundaries.