I am guessing wide riser bars are a combo of necessary bike control and that it's the "in look." Just like narrow little risers were 15 years ago.

As a singlespeed cross racer, there's a reason tracklocross races are completely separate from "regular" cyclocross. You really can't ride a cross race fixed. The turns, off camber hill rides, and dismounting-mounting over barriers would be super difficult with a fixed drivetrain. Tracklocross races are flatter, more open, and closer to XC mtb racing than cyclocross. I also see a lot of nice, dry California tracklocross races and less sleety, muddy northern races.