If a central bank were to react in an asymmetric manner, namely only with looser policy at times of asset price busts but not with tighter policy when asset price bubbles emerge, the central bank may create through its own behaviour a moral hazard problem among market participants.
It is crucial for a central bank to avoid this, since a perception that it insures investors against the risk of large losses could easily contribute to the formation of new bubbles in the future. All these considerations underscore how delicate is the issue of how to best communicate with the public during episodes of sustained asset-price movements.