I'm betting on the fact that your brake pads are too big for the brake caliper attachment. I had gotten some aftermarket inexpensive brake pads (good mind you, just not OEM for 70$) and what they seemed to have done is machined the pads for OEM size (reverse engineered) and THEN painted over them, increasing the size such that they no longer fit in between the springs. I shaved this down a little bit with a grinder (just enouph to get through the paint) and they slipped in correctly. If the brake pads are hard to put in when you replace them the brake pad may just not be moving on the outside so that when you press the brake it only pushes in the inside brake pad. This might cause an increase in pressure therefore causing the rear inside brake pad to wear more. I'm no expert, but from just intuition and previous knowledge thats what occurs to me. hope it helps!