  In this short communication we present a simple internal mechanism that
accounts for the recently observed anti-glitch in magnetar 1E 2259+586. We
propose that the decay of an internal toroidal magnetic field component would
de-stabilize an originally stable prolate star configuration. Then, the
subsequent rearrangement of the stellar structure would give rise to a "more
spherical" configuration, resulting in a sudden spin-down of the whole star. We
present here some order of magnitude calculation to give confidence to this
scenario, using the simplest analytical stellar model and let more detailed
calculations for a more technical future paper.
