There needs to be a logical reason why a Ta'us, a mistake, is a sign of something bad. A mistake comes from a lack of concentration, and to avoid making a mistake in all of Shemoneh Esreh requires a huge amount of constant concentration, which the Chachamim did not require of us (i.e. they allowed for the possibility of making a mistake). However, we can be expected to concentrate hard for a limited amount, such as for a single Berachah. If, during that period, one still makes a mistake, then it is a Siman Ra.

To say that making a mistake in any of the Berachos of Shemoneh Esreh is a Siman Ra would need logical support, for we would think that making a mistake, although it is not good to do, is not a sign of maleficence. (Even if you might argue that making a mistake anywhere in Shemoneh Esreh while one is standing before Hash-m is so serious as to be called a Siman Ra, but since you can argue either way, the Gemara's question is justified.) Hence, the Gemara asks where in the Shemoneh Esreh does a mistake constitute a Siman Ra, and it answers that in the Berachah of Avos it is a Siman Ra, because of the logical reasons that Rashi and others (such as Maharsha) give.

Y. Shaw