MTB tires: why the extended thread on the edges? I have tires similar to this one:


My point is that the tire is quite "flat", with large knobs on the edges, unlike most MTB tires, which are round.

The tires feel quite unstable on any hard surface when cornering. What is the design reason behind making an MTB tire non-round?
These are downhill MTB specific tyres. The extended side knobs will fully engage when cornering hard. By hard I mean:


http://www.pinkbike.com/photo/6705085/
http://i129.photobucket.com/albums/p223/XCSlayer/DSCI4707.jpg


The large gap between the center and the side knobs is mainly to reduce weight and to improve the mud management of the tyre (to not allow mud to be blocked and carried between the knobs).

A review of the specific tyre you posted can be found at http://www.pinkbike.com/news/Maxxis-Minion-High-Roller-2-review-2011.html and for the side (cornering) knobs it says:

The cornering knobs were altered with this in mind, angling them slightly
for more bite and adding a vertical sipe, again for added knob flex to 
allow them to conform to the ground better than a solid knob. Changes 
were also made to have the cornering lugs bite sooner than on the 
previous design.