1. It's harder to build trust than to demolish it. Judging from the timeline, it would take about 2 decades like the 90s to rebuild the three sharp declines.

2. One could identify the sharp declines with significant events (Covid lockdown 2020, War on Terror campaign from 2003, but I fail to see what occured in the late 80s)

3. Still, there is an overall decline and I would look at the hierarchy of trust that exists in a society. Institutions are abstractions of different aspects of the living communities. But since individualism is rampant in our way of life, the whole chain of trust starts to slowly crumble.