The Plague!

In the 14th century, Europe was relatively stable and prosperous. Italy was involved in international trade and its powerful City-States had their own individual and thriving economies, including everything from maritime trade, banking, and textiles (Gardner's Art Through the Ages, Vol. 2, 12th edition, pg 402). However, the Black Plague, which came from China, swept quickly across all of Europe, killing off 25% - 50% of all Europe's population (some of Italy lost up to 50% - 60%) (pg. 402).
I believe that the Black Death had a direct play on the uprising of Humanism in art. Humanistic thoughts and beliefs came from reason, instead of traditional religious views and traditional authority. Think about it... after losing half of your friends and loved ones, one has to consider reason and start asking questions of how and why did these terrible things happen.
Giotto di Bondone started thinking humanisticly in the early 1300's, even thirty years before the Plegue occured in the 1340's. In my mind, he gave the Italian Renaissance an early jump start. He is known as the father of Western pictorial art. "Regardless of the other influences on his artistic style, his true teacher was nature-the world of visible things"(pg. 408). His inspiration was observation, and believed "The visual world must be observed before it can be analyzed and understood."(pg. 408).
The Plague took its toll on artists hearts and minds. So, they started observing what was going on in the world around them, and started painting what they were observing. Therefore, art became more grim and dark in its subjects.