howd they pick redwood?
Generally callsigns are something relative to the airline, or something unique to the airline. I don't think they could use 'Virgin' just because of the issues stemming from US laws and relating it to the int'l carrier.
In this case, they did the same thing AWE and ROK did. AWE is based in Phoenix, and that area (let alone all of Arizona) is known for Cactus, so there you go. National was based in Las Vegas, and didn't want to be related to the old National Airlines and Sunking, so it picked something unique to Vegas, which was Redrock Canyon; hence, Redrock.
Virgin America? it's based in San Fransisco, and Northern California is known for the redwood trees growing all over the place; So, Redwood.