Cause of uneven Brake Magnet wear

Hi guys,

I'm new to the forum and just purchased a new to me 2006 2720. I was going through the trailer, cleaning, checking brakes, packing bearings etc when I noticed that the electric brake magnet is worn funny and the inside face of the drum has some pretty deep grooves. I searched and searched and have found mention that this can happen and that the magnets need to be replaced and the drums need to be turned or replaced but I can't seem to nail down why this uneven wear happens and how to prevent it from happening to the new parts. The shoes seem to be in great shape and the braking surface of the drum is nice and even. I guess I'm surprised that a 3yr trailer would have that type of magnet wear.

Any insight you may have would be great!

Thanks