SENTENCE USAGE: Ol’ Joe Bob hadn’t had a drink in 10 years, but when his rabbit dog Blue up and died, he fell off the wagon.
What does drinking have to do with "falling off of a wagon"?
The phrase "falling off of the wagon" is used when one resumes an addictive/compulsive behavior they are trying to control. Originally referred only to drinking, "falling off the wagon" is now used in reference to all sorts of things including drugs, smoking, overeating, eating certain foods, etc.
It originated with a fixture of America’s past, the water wagon. Before roads were routinely paved, municipalities would dispatch horse-drawn water wagons to spray the streets in order to prevent the clouds of dust traffic would otherwise cause. Anyone who had sworn abstinence from alcohol (and would presumably be drinking largely water from then on) was said to have "climbed aboard the water wagon," later shortened to "on the wagon."