If the rim isn't wide enough and you run it at a super lower pressure you put more wear on the tire through a rotation on the outer-edge of the wire-bead. The rim pushes into the tire and this rubs the outer sidewalls. If the rim is fairly wide there is less twisting of the bead because the rim sits further out and doesn't sink into the tire, instead it sits on the stronger sidewalls.

Anyways, hope that makes sense... might explain why you're having these annual failures, unless you are racking up a looot of miles!