We investigate whether the recently suggested rotation and crystallization driven dynamo can explain the apparent increase of magnetism in old metal polluted white dwarfs. We find that the effective temperature distribution of polluted magnetic white dwarfs is in agreement with most/all of them having a crystallizing core, and increased rotational velocities are expected due to accretion of planetary material that is evidenced by the metal absorption lines. We conclude that a rotation and crystallization driven dynamo offers not only an explanation for the different occurrence rates of strongly magnetic white dwarfs in close binaries but also for the high incidence of weaker magnetic fields in old metal polluted white dwarfs.