F=muN only applies when the two surfaces are in contact. Bumps, debris, cracks, etc. all cause higher pressure tires to rebound with more apparent force due to the conservation of energy which states that, in a closed system energy only ever changes forms. In a higher pressure tire, the energy with which you hit a bump returns to the tire in such a manner that the tire skips and contact between the two surfaces is momentarily lost, so F=muN collapses to 0 (the tire is essentially in the air; in the case of sand you've then changed the surface and thus the coefficient of friction, or mu in the equation). When a lower pressure tire hits a bump, the tire conforms more to the shape of the bump and energy is conserved in the form of momentum and heat. So the issue then is one of whether or not the tire is actually in contact with the ground surface, lower pressure tires are better at this than higher pressures.

It's not exactly that simple, but that's the gist. You may then ask "why are dragster tires so wide then...?" That is just a matter of applying forces over a larger area so that the tires can actually be used, because the tires themselves are soft and sticky.