It could be something as simple as the two cats looking out the window together. One sees a bird and gets excited, the second goes to sniff the first and gets an evil look, a hiss or a pawful of claws in the side of the head (redirected aggression). Then the second cat is scared for a couple of days, and the harder you try to make him feel better, the more afraid he gets.
My cat's sensitive to barometric pressure, gets very standoffish and irritable when the barometric pressure drops, and she goes completely bonkers during windstorms and the spring thaw when snow and ice slide off the roof. And, occasionally, she just goes "psycho-kitty" for no apparent reason (at least not one apparent to my dull human senses).
Playing gently often helps a cat forget a sudden skittishness for a time. Also, a low-volume radio or TV can help even out the peaks and valleys in daily noise that might bother a spooked cat.