I say normal wear also. I think it has to do with braking forces on the tread blocks.
I've seen it (always) on my knobby tires (especially TKC80) and I'm quite careful with my tire pressures.
On my bike (DR650) the wear pattern makes handling pretty squirmy on grooved pavement and fast, but bumpy, corners. I don't get very good mileage on those tires because the handling gets so annoying. They're great in the dirt, though.
............shu
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