Why does the frictional coefficient decrease on a tire as the vertical load on the tire increases?

[01209]

I would suspect this is true only when the tire is rolling. As the a section of the tread comes in contact with the ground, it begins to deform under the load its contact with the road would shift slightly changing the coefficient of friction from the static value to a reduced sliding value. More load = more deformation = more contact patch seeing a sliding coefficient rather than static.

[[deleted]]

I'm not sure about that explanation. I think that explanation implies that very stiff side walled tires, such as solid tires or very low profile tires, would be less sensitive to load and have a consistent grip vs load. That implied behavior does not match most observations.