Low interest rates prior to the crisis enabled build up of leverage. In fact, if we observe closely, leverage happens to be the major contributing factor to most crises. Let me explain in brief.
When times are good, there is a feel good factor, there is plenty of liquidity which is cheap, the asset values and collateral values are high and the banking system losses are low, leading to lower provisioning and lower capital requirements. These factors lead to higher demand for credit from the households and corporates.