This is what is called a Pimp Invasion. Basically, you call up "Pimps 'R Us" (5557467) and head a good bit away from where the Pimp you called stops. Then, in the districts where they normally appear, (even, sometimes, the Airport, Downtowm, and Saints Row districts, but this is rare) there will be hundreds of pimps around, regardless of the time of day