Bailing out of a debate is not unheard of in politics (hey there, John McCain).
But it is weird in this context: John Koster, the Republican candidate for the U.S. House of Representatives in Washington's 2nd District (Everett, Bellingham, and the San Juan Islands), is refusing to have a live televised debate—the only lived televised debate scheduled close to the balloting in this contest—because he doesn't like one of the proposed debate panelists, a reporter who works for a newspaper that endorsed him.
So weird it requires a theory—especially since Koster isn't explaining himself. I have a few:
1) There's some amazing beef between Koster and Everett Herald reporter Jerry Cornfield that no one knows about (including Cornfield's own editors).
2) Koster is, as the campaign of incumbent Democratic congressman Rick Larsen says, "trying to hide his extreme views from the voters of Northwest Washington." This may sound like a gratuitous attack, but in fact Koster does have some pretty extreme views. For example, he opposes abortion even in cases of rape or incest and wants to do away with the Department of Education. He also comes off as kind of angry on television. Maybe Koster and his campaign think it's better to keep the anger and the extreme views off the air in the closing weeks of the campaign.
3) Koster knows something no on else does. He knows, either from internal polling or some sort of psychic connection to the voters of the 2nd District, that the Republican wave is huge this year, and all he needs to do is shut up and ride it. After all, he won the primary, relegating Larsen, a five-term incumbent, to second place—and that was before any televised debates. Maybe his internal polling and/or psychic connection tells him he can have a repeat of that performance if he just stays off the air.