Several reasons: Coincidence, as craigsmom says; cars are often parked under trees, eps. when it's hot; open commercial parking lots attract seagulls, pigeons & sparrows looking for discarded food scraps; birds see their reflection in car side mirrors & attack. (They leave poop dribbled down the side of your car. Current problem w/ mockingbirds in the yard. Ick...)
If birds just randomly drop, then why don't we see people walking around with bird droppings in their hair or on their clothes? There has to be a bird coalition that seeks out cars. LOL! I seriously think they seek my car out, especially after it has been washed. The feather offenders seem to know when my car is clean and shiny! I've learned not to park under trees where birds are hanging out, even if I am tempted to because of the shade.