As I noted in another post back in November American companies have been loading up on debt the last few years. Right now the amount of Corporate debt outstanding is so big it is at an all time high if you compare it to GDP.
Corporate Debt to GDP
This is possible because the yield on corporate bonds has been steadily declining to all time lows the last few years.
Corporate Bond Yields
What is staggering is that this has happend at the same time while corporate profits as a percentage of GDP have never been higher.
EDIT (April 3, 2014): Please read this article regarding the relevance of the CP/GDP chart.
Corporate Profits As A Percentage Of GDP
Meanwhile all this extra money has not been used to massively invest in new products or services. Companies have been laying of a lot of workers the last few years also. This can be seen in this chart below which depicts what percentage of corporate profits has been used to invest.
Net Domestic Investment (Private Business) To Corporate Profits
Instead companies have been using all this new money to buy back their own stock or to pay a dividend to shareholders. Activist shareholders like Carl Icahn have been pushing companies like Apple to give money back to shareholders instead of investing it in new products.
I came across this chart a few days ago on Twitter. It shows that when the use of margin debt is high relative to GDP, average 30 month returns on the S&P 500 will be poor. This chart suggests that returns on the S&P 500 will be negative the next few months.