The collapse of Lehman—itself a symptom of many other problems—played the role of trigger (see the Appendix). Returning to our earlier example of household expenditures that depend negatively on the past accumulation of consumer debt, it is easy to imagine how this may be activated by a trigger.
For example, the trigger may be a repricing of risk on international markets that causes a change in long-term mortgage rates. As long as the trigger is dormant, the negative effect of the vulnerability is muted; but this effect comes into full force once the trigger activates it.