What caused the economic boom of the Roaring 20's to happen?

I'd also have to add the overwhelming infusion of cash into the
America economy from the arms industries and the production of war material for World War I.  $17
billion worth of arms sales to European empires took place at that time, and this created not
only massive wealth in the hands of a few industrialists, (which they were able to invest later
in new economic ventures) but drove up wages, caused increased consumer spending, and increased
stock prices.  The economy expanded greatly as a result, as did its industrial
base.

Republican laissez-faire administrations over the course of
that decade that kept taxes and regulations low sure didn't hurt, but I believe they merely
accelerated an economic trend already brought about by the vast fortunes of war, and a new
American middle class who wanted to share somewhat in the luxury of the new
economy.

From the first link
below:

Annual incomes
rose steadily, from $580 in 1914 to more than $1,300 by the end of the decade. Thus, World War I
dramatically changed the domestic and international economy and set the stage for the prosperity
of the 1920s.