In high school, after I decided life was an anthropology experiment, I set out to understand how people react. Answering the question why they react a certain way is far harder, so for the most part I don't even try. Understanding how a person (or anything really) will react to a given stimuli is important in the future so that we can build effective relationships and not make the miskates of our past. By observing what actions obtain positive and negative reactions we can learn what to do and what not to do.
Extending this concept to economics, when S&P downgrade the US credit, the stock market takes a hit. Going down for a few days is how the stock market reacted. The why is that investors felt that trouble was eminent and there were safer place to put their money than stocks. Downgrading of a country's debt rating implies that that country is not totally stable enough to pay their bills. In other words, the country has a combination of weak economy and not absolutely effective government. Which means that a recession is possible. Even in that simple example the why is more complicated.
One more example is failure of a structure. Why it failed has to do with design inefficiencies or an unplanned load case. How it failed is the physical phenomena of material failure analysis.
Another example, perhaps someone yells at you after you forgot to do the dishes. That is how the person reacted to your dish doing failure, but chances are there is something else going on with that person that caused him or her to yell. Understand why a person yells or acts a certain way is far more complex. Although, in anoint term committed relationship the why is probably helpful to understand as well.