I'm not knowledgeable in Pacific Salmon, but this is a phenomena seen in lots of species, and has a fairly basic biological explanation. A species can be exposed to individual threats that they can still cope with and adapt to, resulting in little immediate decline. But as more of these threats accumulate, eventually the sum of the whole overwhelms the species, and can lead to a sudden drop.

A steep drop can follow a more gradual decline, as well, due to loss of genetic diversity in the population making the population less able to adapt to another unrelated threat. Check out the Extinction Vortex.