The backs tend to last longer cause
1. they're hard to change.
2. they squeal like a banchee so they aren't used as often.
Cause for squealin', dirt and maybe moisture.
fixes:
1. clean (it don't stay clean for long)
2. get them hot, real hot, slowly
3. don't fix them, use as a very anoying claxton to let people know you're braking. And if you're anoying me by causing me to brake, I'm going to anoy you too. Also makes a nice wolf whistle in combo with front.
BTW: EBC red's minimize rotor wear and don't squeal at all.