They are afraid of the dark. Since their fur basically looks like and glows like fire, they are light sources (of various brightness depending on their moods and health) wherever they go and thus don't really know what true darkness is.
Humans fear the dark because of the unknowns that may be hidden in its shadows. Bookas fear the dark because it IS the unknown.
Gesturing aggressively at Anne McCaffrey with one of those little stick things people use in lecturs to point at the chalkboard: So Anne McCaffrey, if the reason your queen dragons choose women is so that your Dragon Riders don't have gay sex because you hate gay people even more than you hate women, what, exactly, have the riders of the blue and green dragons been doing this whole time whenever a green dragon is in heat, huh, Anne? Answer the fucking question Anne!!!!
Booka is the best thing in Slewfoot. No arguments will be made. Do I wish I, too, had a farm in the middle of the woods with bees and goats and could live peacefully? Yes. Do I wish I had a beastman as my bestie who helps me do magic? Also, yes. But none of this negates the fact that Booka the cat is the best part of the book, and we were all ROBBED when Brom took him from us