Demographic Crises
The Great Famine
It's worth establishing that there were at least some signs of trouble before starting in on the long, grim tale of demographic crisis that makes up most of the population history for the next two hundred years. These catastrophes did not strike a body that was in peak health. It wasn't sickly, by any means, but neither was it fully healthy.
The first crisis that struck wasn't the plague, it was famine. There had been famines before in Europe, of course. Plenty of regional ones and even a few general famines. But what happened in the 1310s was unprecedented either before or after in European history. In all, the string of famine years killed off perhaps ten to fifteen percent of the population, with some areas suffering very badly indeed.
The famine happened only in the north. Ironically, the 1310s were exceptionally good years in Italy and elsewhere around the Mediterranean. The disaster affected northern France, Germany, the Baltic coast (including Scandinavia and Prussia), and the British Isles. Austria seems to have escaped for the most part, though Styria was affected. We know much less about eastern Europe.
Famine can happen for a variety of reasons, but in this case it was clearly the weather (conditions in some regions may have been made worse by the disruption of warfare). Specifically, it was heavy rain and extreme cold. Incessant rain, lasting from planting right through harvest, so that the crops were ruined (especially wheat and beans). This was followed by winters that were exceptionally harsh and cold. The snows persisted late, further disrupting the crop cycle. Such weather had happened before, but rarely both together and never for more than a single year.
Three Bad Years
The bad weather began in 1312 and 1313 but was mainly in Germany. The general bad weather came in spring 1315, when the rains began. The following winter was even colder, and the Baltic Sea actually froze. The following year, the rains came again, relentlessly. Flooding was everywhere. Between the rains and the cold, orchards were suffering as well as the annual crops. Most of the food reserves had been used up in the previous year and now famine was widespread across northern Europe.
That winter was bad again, but the next year at least the rains relented. The chroniclers report, however, that the harvest was very poor. The winter was better, but in 1318 the rains came yet again. After that, bad seasons persisted into the 1320s. Normandy saw terrible windstorms in 1319. Flanders flooded again in 1320, the Baltic froze over in 1321, and Flanders again experienced widespread flooding in 1322. The worst years, though, were 1315 through 1318.