Gigi, i've got a good article coming out on that very subject, but the best answer i've arrived at comes from this area:

TLDR, past a certain point, the systems move from satble/sub-critical, and past that point, any single addition grain of sand (or snowdrop on an avalanche, or unit of currency), could do nothing at all, or trigger the whole slope to collapse.

https://guava.physics.uiuc.edu/~nigel/courses/563/Essays_2012/PDF/banerjee.pdf