>An unconventional monetary policy in which a central bank purchases government securities or other securities from the market in order to lower interest rates and increase the money supply. Quantitative easing increases the money supply by flooding financial institutions with capital in an effort to promote increased lending and liquidity. Quantitative easing is considered when short-term interest rates are at or approaching zero, and does not involve the printing of new banknotes.
So if suddenly, a new player shows up in the markets buying trillions of dollars worth of securities, due to the law of supply and demand, one can expect price of stocks to go up. You can check out the Feds balance sheet here [1]. Check out "Securities held outright" [2], yeah, that's 4.3 trillion with a "t". This figure used to be on the billions before the financial crisis [3], which means that the Fed flooded secondary markets (stock exchanges among them) with more than 3 trillion dollars, to put this into perspective, this the value of all the goods and services that the US produces in 2 months, or two and a half years in the case of Mexico.
So yeah, the causality between QE and inflation of stock prices seems very plausible.
Disclaimer: Central banking operations is not one of my main strengths, so you are more than welcome to fact check, correct or complete my comment.
[1] Web: http://www.federalreserve.gov/releases/h41/current/ PDF: http://www.federalreserve.gov/releases/h41/current/h41.pdf
[2] The amount of securities held by Federal Reserve Banks. This quantity is the cumulative result of permanent open market operations: outright purchases or sales of securities, conducted by the Federal Reserve. Section 14 of the Federal Reserve Act defines the securities that the Federal Reserve is authorized to buy and sell.