They did not stop building homes in San Diegeo County. Houses did not disappear in San Diego County. Mexicans did not stop pouring into San Diego County. Birth rates are not down in San Diego County.
Something is wrong with the census data. You can take that to the bank!
Something is definitely wrong with their counting. With such an exodus, housing prices ought to have dropped and those new housing units should have had to be auctioned off or offered for attractive leases; these did not happen. Supermarkets and other stores should have been closing, which I haven't noticed. Traffic didn't get any better, either.