There is a lot to know about truck tires and tread wear patterns, and I'm no expert. Not so long ago, I would have said the same thing Greg said about your tires, underinflation. On car tires it would be a good bet, but on truck tires, it may or may not be. That is because truck tires are designed differently and behave differently under varying conditions. Car tires are less subject to such variables.
For example, on one truck, it may make perfect sense to run the steer tires at the inflation printed on the side of the tire, say 110 lbs. On another truck, with the same steer tires on it, it may be better to run at 80 p.s.i. Differences may include load characteristics (weight on the steer tires), the speed at which the driver normally operates over the road, the wheel cut built into the truck (how sharp the front wheels will turn), and more.
As I continue to study truck tire characteristics, I am learning that OVERINFLATION may cause the problem you describe. To confuse things further, if you use the number printed on the side of the tire to determine your proper inflation level, a steer tire inflated to 110 p.s.i may seem to be properly inflated. But a look at your particular tire's load table may reveal that 90 p.s.i. is best for the load your steer tires generally carry. Further research may indicate that an entirely different tire may be better suited for you.
If your steer tires are designed for heavy loads and you are running them at maximum pressure, but if you are carrying lighter loads than the tire was designed for, you will not have an even footprint across the surface of the tire as it contacts the road. The center ribs (treads) will contact the road well but the outside ribs will not. This may produce the irregular wear pattern you describe.
As Goodyear puts it,
"If a tire is highly loaded, it tends
to have a square footprint shape. The
shoulder rib contact area is very long,
about the same length as the center ribs.
As the tire rotates, contact with the road
is good.
"By contrast, a lightly loaded tire tends
to have very short shoulder ribs, much
shorter than the center rib. As this tire
rotates, the footprint center maintains
very good contact but the shoulder
area does not. This causes much more
scrubbing action and wearing away of
the shoulder rib."
Some of this is counter-intuitive but it makes sense as you think about it. It takes a little work to get familiar with terms like shoulder ribs, crown, sidewall, etc. But the work is worth the effort if you want to "dial in" your tires to your particular truck and the way you drive it.
A helpful publication is free online at:
http://www.goodyear.com/truck/pdf/radialretserv/Retread_All_V.pdf