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, often shoes. It is also linked to the fact that when you are depressed you look down at your shoes and decide they aren't good enough quality so buy more expensive replacements. 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 spiralling into ruin. In the case of Brontitall and Frogstar World B, the population forsook shoes and evolved into birds.
Brontitall
On Brontitall, the Shoe Event Horizon was exploited by the Dolmansaxlil Galactic Shoe Corporation who made shoes that were always too wide or in some way would never fit anyone so people were always replacing them. This is why Brontitall, whose inhabitants have now evolved into birds and will never mention shoes, has a geological deposit of compressed shoes. The deposit dates back to when the civilization, which is now made up of birds, once existed on the ground. The Dolmansaxlil Corporation now guards the ground of Brontitall using Foot Warriors, armed robots whose shoes do not fit, causing them to always limp.
Background
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).