Can someone explain what caused the wall street crash ?

The 2008 Global Financial Crisis was the result of risky lending practices and a reliance on certain financial products that were faulty and were built to eventually fail. Banks had leveraged too much debt into the economy through risk "derivative" type investments that over time led to increased instability and eventually the collapse of major U.S. financial institutions like Bear Sterns and Lehman Brothers. In addition major insurance groups like AIG and others were also put at risk and many major banks and insurance companies received massive bail outs from the U.S. Government.