photos are from a parade in Houma Louisiana. For this one, people on the floats throw vegetables. It fetches back to the original origins of the mardi gras parades.

A music band would assemble and follow a wagon. Just before lent began, people would put their left overs on the wagon and collectively have one last meal. The forty day fast that followed mardi gras allowed the grasses and lettuces to sprout and develop such that people didn't graze their new crops right down to the dirt. So the legend would have it at least.

EDIT: Except the first photo. That is from some california demonstration that I happened past. Didn't really stay to discover the message.