This is a common thing with "tready" dirt bike tires.
Both my old 950 Adventure or XR650R both wore out the side treads first, and now my Caponord with TKC-80s.
When the tire rolls the center tread has a smaller surface area and just gets pushed into the tire. Where as the side treads as a team have a greater surface area and take the load of the weight.
If you air up the the tire too much then you will not have as much surface area contacting the ground, the tire will wander and it wont be good.