The most obvious answer would be due to the bubonic plague. It killed off about a 1/3 of Europe, and from the uneducated point of view of those present, they took it to be a sign of impending Judgment Day, or some other punishment sent to them from god. The actual definition of apocalyptic does not have to be the strict christian view of it either. it actually means: involving or portending widespread devastation or ultimate doom. The Middle Ages were certainly devastating and terrifying to the people.