I have always wondered that question, so I took a moment to see if I could find any literature on it quickly. There is (or was?) a long-term study in the hybrid contact zone in Riske Creek (west of Williams Lake) and I found a paper from this research that seems to cover or allude to the reason. The paper can be found here:
http://www.collectionscanada.gc.ca/obj/ ... 150525.pdf
The line I read that seems to hit the nail on head is that 'Yellow-shafted' Flickers winter east of the Rockies, while 'Red-shafted' winter west of the Rockies. While "red-orange" hybrids winter among 'Red-shafted' Flickers in the west. It would be interesting to pay closer attention during the breeding season to see if we exclusively get these hybrids during the winter or whether we are seeing influences of the contact zone bleeding over to the coast.
Thanks for putting that question out and hopefully a more thorough read (as I just quickly sought out an answer) will further elucidate the answer.