These events typically have a lower frequency than the business cycle frequency to which our normal policy practices are attuned. They build up over a number of years. Furthermore, asset price movements and their effects are often asymmetric. When asset prices are rising, they can easily co-exist with low CPI inflation.
But because this is often associated with a build up in debt, the ensuing correction in asset prices can cause acute distress to borrowers. This means both that spending behaviour is affected and that there can be a risk of systemic financial instability because distressed borrowers often mean a deterioration in the asset quality of the lenders.