Hi Mongol,
As you say, this wear pattern on Unimog's is quite common, I'm not sure why this is the case but it does vary with the tyre, the worst I know of is the Michelin XML.
I can hazard a guess, but Dad may know better.
One of the thing's it could be is the unimog has a natural positive camber, this is to allow for a certain amount of load on the front axle (due to it's portal axle and disk brakes there is a lot of leverage there) hence, when there is no weight on the front, the top of the tyre sticks out further that the bottom of the tyre. perhaps causing uneven wear.
The second thing is the offset of the unimog wheels, because the hub face is quite a distance from the kingpin on a Unimog axle (due to the portal hub) the wheel forms a natural arc when turned. this could perhaps cause tyre scrubbing.
The third theory, and one which I think may be correct, or at least part of the reason, is the tyre itself. I know I have seen mancats with XML tyres which show the uneven wear in the past, so perhaps it is just a characteristic of the tyre.
Cheers, Brian