The gemara states that the reason for not blowing shofar on Shabbos (and not taking the Lulav, and not reading the Megilla) is because one might come to carry it in the public domain, thus transgressing a Torah prohibition. This might even be only an unintentional act (shogeg), such as the person forgetting that it is Shabbos. The gemara says that since the person is involved in a mitzvah but is not necessarily an expert in how to perform the mitzvah, he may have a question about the mitzvah and go out (rush out without thinking perhaps) into the public domain with the item in his hand in order to ask his Rabbi a question about the mitzvah. The Sages considered this a serious enough concern to decree a complete cessation of the mitzvah of shofar on Shabbos.
I hope this is clear and helpful.
