it is an old tradition.
http://en.wikipedia.org/wiki/Wedding_reception
A ceremony is often made of the newlyweds' departure. Rice or birdseed, signifying abundance, may be thrown at the departing couple, with birdseed preferred by facility managers, since it requires less clean up work than rice, and new, mess-free substitutes, such as blowing soap bubbles or ringing small bells being even more favored by the cleaning staff.[5]
As the newlyweds are the guests of honor, the other guests are expected to remain at the reception until they leave them, and consequently, it is an imposition on the other guests for the newlyweds to stay unreasonably long at the party.[5] On occasion, the newlyweds will stage an official leave-taking, so that guests feel free to leave, and then quietly return through another door.
I remember going to my aunts wedding as a kid and sleeping in the car. (my sister was flower girl).