Fill ground and settlement
Much of the flat land along the bay in northern San Mateo County is low-lying and, in places, fill placed over soft bay mud. Ground like that settles, and it settles unevenly.
Pavement follows the ground. Low spots form and hold water, and once water is standing on the surface it works into the aggregate base and turns a settlement problem into a structural one. Correcting the slope is usually cheaper than repairing water damage over and over.