EM points to a Carbon Brief report on the causes. The reports quotes a Brazilian Government Researcher as saying

In normal years, most of the rainfall feeding the Southeast is carried from the Amazon through the aerial rivers. The link of deforestation with reduced rainfall within the Amazon is well established. One needs only to connect the dots.

When I looked at the actual air flows prevailing in 2014, it seems that the cause of the drought was a southward shift in these prevailing air flows. Instead of this winds going westward from the Atlantic over the Amazon, they were over the much drier states of Bahia and Mato Grosso.
If deforestation has affected the local rainfall, why does nobody mention local deforestation? The South Atlantic forest used to extend from Rio to the north, to Paraguay, and Argentina well to the South. Once covering an area larger than Western Europe, about 90% has now disappeared, compared with about 22% of the distant Amazon.
The study quoted by Carbon Brief only goes back to 1941. The worst drought on record was in 1930.
The severity of the drought in terms of water shortages might have something to do with the major reservoirs being developed in the 1920s and 1930s, when the population was less than a tenth of the current 21 million.
The Sao Paulo drought is a clear illustration of the two sides in the global warming "debate". You can either try to understand the actual evidence, or side with the alarmists, like Entropic Man, who look at the opinions of opinions, and try to direct you away from the actual evidence in favour of prejudiced opinion.

Kevin Marshall, don't confuse climate scientists with factual evidence, especially if it involves documented historical records. It goes against all their ingrained belief systems.