or he could have activated yata's mirror independenlty of susano'o when he realized what sasuke was gonna doHow was susanoo an automatic defense? A hell of a good defense, yes. Automatic activation? Questionable logic there. Especially, since it would have to automatically activate against and be faster than a one-shot jutsu that Itachi hasn't seen before that happens 'at the speed of light.'
point is we'll never know the exact details but whats important is that it took one of the most powerful defensive items to block kirin