There are of course many potential sources of tail risk. It could be financial institutions engaging in increasingly aggressive maturity transformation. Or it could be increasing bank exposures to a particular sector or asset, say the housing market. In general, a necessary condition for severe crisis seems to have been excessive leverage.
For any such potentially risky development, there is typically no shortage of good explanations why everything is, in fact, fundamentally healthy. And obviously, the central bank does not really possess superior knowledge. What is clear though is that aggressive expansion of credit increases systemic risk and, ultimately, the risk of a breakdown of monetary policy transmission. Hence there has to be a policy response.