As you look at different tread patterns, you will find that essentially all of them can be broken down into blocks of rubber, rather than straight grooves all the way around. The idea behind these blocks is that they stagger the grooves so the tire is less likely to react to rain grooves, bridge grates and road seams as you ride. The problem arises when you hit the brakes. The rubber in the blocks moves slightly to the rear under the force of braking, which wears the front, braking edge slightly more than the rear edge. This is the beginning cupping.
Cupping is a perfectly normal tread wear pattern. Still, there are things you can do to help reduce the amount of cupping. One is inflation pressure. Inflation pressure will affect how much cupping you get because it affects the flexing of the tire as it goes into and out of the contact patch. If proper inflation pressure is maintained, you may reduce the cupping considerably. On some front tires without directional arrows, you can take the tire off and reverse the direction to even out the wear. But when all is said and done, if your tire is cupped, that tells me that you know how to ride a motorcycle. If you got 20,000 miles on a front tire and it's not cupped at all, you probably have not been braking properly. 80% of the braking should be in the front. With automobiles, tire rotation helps to even out the wear patterns between front and rear. But rotation is not possible on most motorcycles, due to the different size of the tires.
Other wear problems. If you notice that the two outside edges of the tires are worn more than the middle, that should tell you that the tire has been run under inflated or overloaded. As the tire comes into the contact patch, it will depress in the middle and will wear more heavily on the outside edges. Over inflation will, of course, cause rapid wear down the middle, although this wear pattern can also be caused by high speed burnouts. You'll also see motorcycle tires worn more on the left side than the right. This could be due to riding on crowned roads where you're riding more to the left side of the tire than on the right. People that live in remote and rural areas tend to ride on more crowned roads than people that ride on freeways all the time. Also, left turns are larger turns than right which contributes to excess left side wear.
Another problem becoming more predominant in the heavy touring market is channel cracking. In the grooves of the tread you will find little cuts that look like somebody took a razor and cut into the tire. Overloading and under inflation and poor suspension are the chief causes of channel cracking. This can be dangerous.
You should inspect both tires for channel cracking. It's a common problem. Suspension and inflation pressure checks become especially critical on heavily loaded touring bikes.
RIDE SAFE