I do not understand why there are so many Oregon fans in the Bay Area, so say nothing of why they are so assertive about it.
Firstly, it is known that a massive chunk of graduates from colleges in Oregon head directly to the Bay Area after graduation. They naturally develop some kind of homesickness (for their college home, I mean: many of them were from the Bay Area to begin with) and this may present as wearing duck hats and green and yellow all the time. (I headed directly for Wisconsin after graduating from my Oregon college; I felt the deprivation of mountains very very intensely, pathologically.)
Secondly, this is just one manifestation of a cross-cultural phenomenon. There are many Scotland soccer team fans in London (probably more than England fans these days), and they're known for being "assertive." Ditto with Bretons in Paris: watch some Benoit Hamon rallies and when you see the Breton Stars and Stripes flag* you'll know what I mean.
*What I call the flag of historic Brittany - but the 'stars' are actually weird little emblems, probably ancient and Celtic in origin.