Causes
There were five main reasons for these mass uprisings including 1) an increasing gap between the wealthy and poor, 2) declining incomes of the poor, 3) rising inflation and taxation, 4) the external crises of famine, plague and war, and 5) religious backlashes.
The first reason was because the social gap between rich and poor had become more extreme. The origins of this change can be traced to the 12th century and the rise of the concept of nobility. Dress, behaviour, manners, courtesy, speech, diet, education — all became part of the noble class, making them distinct from others. By the 14th century the nobles had indeed become very different in their behaviour, appearance and values from those "beneath".
The second reason was a crisis for the nobles with declining income. By 1285 inflation had become rampant (in part due to population pressures) and some nobles charged rent based on customary fixed rates, based on the Feudal system, so as the price of goods and services rose (from inflation), the income of those nobles remained stagnant (effectively dropping). To make matters worse, the nobles had become accustomed to a more luxurious lifestyle that required more money. To address this, nobles illegally raised rents, cheated, stole, and sometimes resorted to outright violence to take what they wanted.
Thirdly, kings needed money to finance wars and resorted to devaluing currency, by cutting silver and gold coins with less precious metal, which resulted in increased inflation and in the end, increased tax rates.
Fourth, the 14th century crisis of famine, plague and war put additional pressures on those at the bottom. The plague drastically reduced the numbers of people who were workers and producing the wealth.
Finally, layered on top of this was a popular ideological view of the time that property, wealth and inequality was against the teachings of God, as expressed through the teachings of the Franciscans. The sentiment of the time was probably best expressed by preacher John Ball during the English Peasant Revolt when he said "When Adam delved and Eve span, who was then the gentleman?", criticizing economic inequality as human-made rather than a creation of God.
Read more about this topic: Popular Revolt In Late-medieval Europe