Tightening monetary policy too early could plunge the economy back into recession. Moving too late could let inflation take off and require more tightening to get it back under control. It could also result in bigger financial imbalances which, later on, if they unwind, could throw inflation into another downdraft.
The recent market turmoil has also affected the liquidity positions of financial institutions that do not ordinarily have access to the discount window. In particular, prior to the recent experience, it was believed that primary dealers were not especially susceptible to runs by their creditors.