The genetic "unnatural selection" of the aggressive fish seems to make the most sense. In the mid-west, this has been documented with walleye; there are known stocks that don't bite aggressively and others that do. These aggressive and non-aggressive stocks have even been isolated in hatcheries. The average smaller size of Puget Sound coho compared to coastal and Columbia stocks may also be influenced by the heavy historical commercial fisheries in the sound (only the smaller ones got through the mesh). More unnatural selection influenced by fishing.

One other contributor that hasn't been mentioned so far though I think is flow (velocity, not water volume). In my experience, the faster the water velocity you have in a stream, the more likely the fish are to be on the aggressive side. In the Snoho, most of the drifts are large deep glides which move rather slow, compared to the riffle/pools you find farther upstream in the tribs. Likely related to the visibility thing--if the fish sees it coming from afar, it seems to lose interest, whereas if the presentation drifts through the fishes visual area pretty rapidly due to increased flows, the fish is more likely to grab. I've had much better luck fishing higher gradient streams like the Vedder and Sky for coho.

Bringing AK coho down here is a bad idea; these fish are genetically adapted to the cooler climes of more northern lattitudes. Most notably, these fish migrate upstream and spawn much earlier than our fish; down here they would migrate to spawning tributaries in the middle of summer when there is little water and non-optimal temperatures. We futz with the genetics of these fish enough just by fishing for them!