When funds flow from bonds into stocks on a large scale, the yield on bonds should increase and the yield on stocks decrease.

http://en.wikipedia.org/wiki/Fed_model

Competing assets argument

Stocks and bonds are competing asset classes for investors. When stocks yield more than bonds, investors are better off investing in stocks. When funds flow from bonds into stocks on a large scale, the yield on bonds should increase and the yield on stocks decrease, until the Fed model equilibrium is reached.