There are many theories about the cause of drought. I don't know that it does any good to hear them, but there is one theory that we should ignore. That one is the theory that somebody actually knows the cause of such things.
The truth is that nobody knows. When it comes to how the earth's climate-impacting systems operate and interact with each other, what we actually know is like a drop in an ocean.
There is some circumstantial evidence from deep lake deposits, sand dune formations, and tree ring studies that there have been six major droughts in North America during the last 1,500 years (the drought of the 1930s being the last and least severe of those major droughts).
There is some evidence that major changes in climate (even significant shifts in sea levels) have occurred several times in the last thousand years and have done so in spans of less than 40 years.
There is some circumstantial evidence that solar flare cycles (which produce measurable "solar particle events" on earth) have some correlation to drought cycles.
We know a little bit about the carbon cycles, water cycles and the layers of the atmosphere, but we know very letting about how solar events affect those things and almost nothing about how all of them interact with each other. We know very little about volcanic cycles below the oceans, which release unmeasured amounts of heat and chemicals that impact the other cycles.
I have seen many news stories asserting that "global warming" (a buzz term meaning man is doing it) is causing worse hurricanes, floods, droughts and forest fires. I have not seen a news story saying the sun might actually be the cause of global warming. Warming from the sun? What a novel idea!
Well, right now we happen to be in what the scientists call "cycle 23". Here is part of what the scientists at the 29th International Cosmic Ray Conference had to say about that ? The trailing years of solar cycle 23 have provided some of the largest solar energetic particle (SEP) events of the last decade ? The January 20, 2005 event was remarkable from several points of view. It was the largest ground-level event (GLE) measured in neutron monitors since 1956 , and had a very hard energy spectrum. It also was the most intense SEP event measured by NOAA's GOES satellites in their 29-year history (1976-2005). Finally, this event had a risetime that was faster than any of the large SEP events (proton intensity >100/cm2sr-sec with energies >100 MeV) within the last 30 years.
According to NASA, "Variations over the 11-year solar cycle in the intensity of the Sun's electromagnetic output at some wavelengths significantly affect the chemistry, structure, and dynamics of the Earth's upper atmosphere. Longer-term solar variations may be linked to major shifts in the global climate."
Wouldn't it be interesting, if after 5,000 years of study, man were to discover only that the sun (not man) controls all life cycles on earth?
We do not know why droughts occur, but we can't change it anyway. Some think predicting droughts might be helpful, but our predictions are not that reliable anyway.