The Shoe Event Horizon is an economic theory that draws a correlation between the level of economic (and emotional) depression of a society and the number of shoe shops the society has.
The theory is summarized as such: as a society sinks into depression, the people of the society need to cheer themselves up by buying themselves gifts. This is usually done through the purchase of shoes. As more money is spent on shoes, more shoe shops are built, and the quality of the shoes begins to diminish as the demand for different types of shoes increases. This makes people buy more shoes.
The above turns into a vicious cycle, causing other industries to decline.
Eventually the titular Shoe Event Horizon is reached, where the only type of store economically viable to build is a shoe shop. At this point, society ceases to function, and the economy collapses, sending a world into ruin. In the case of Brontitall and Frogstar World B, the population forsook shoes and evolved into birds.
The term may be a pun on the title to the Frederik Pohl novel, Beyond the Blue Event Horizon (although the novel was published in 1980, a year after Adams wrote about it).