Not that long ago at most weddings the guests would tie shoes, tin cans etc to the back of the newlyweds car. The origins of this custom are like the origins of most wedding traditions, steeped in superstition.
In the Tudor period in England guests would throw shoes at the newlywed couple! They considered it good luck if they could hit the carriage or the couple!
Even further back, the father of the bride would pass on a pair of the brides shoes to the bridegroom. This symbolised the passing of responsibility from the father to the new husband, and a variation of this was for the bridegroom to tap the bride on the forehead with one shoe to assert his dominance!! (I can just imagine the scenes in the church if the groom tried this these days!).
The tradition of the throwing of the bouquet originally started with throwing one of the brides shoes over her shoulder.
And the reason for tying shoes, cans and other assorted objects to the back of the carriage? It was generally considered that the guests had to make as much noise as possible as the new couple departed the wedding venue to scare off the evil spirits from bothering the couple, and so bells, horns, fireworks and other things were used to create a racket!