I have two initial thoughts, the first is that you run the tire too low. The tires should be at least the pressure recommended on the placard on your door post or glove box. Often people will pump the tire just to this pressure and not check it again for three to six months. Tires loose about 1 psi per month on average.
Unless you check your tire pressure at least once a month, I would recommend going 2 to 3 psi above the recommended pressure so that the tires will not be under inflated if checked every quarter and only a little underinflated if checked semi-annually.
Second thought is that the alignment may be off a little, either a little too much toe-in or too much camber. Both of these are aggravated by low tire pressure so step one is to check those pressures. You can also rotate the tires to the back of the vehicle where most of the tire wear is in the center of the tire. That will help even out the wear pattern.