Is there any tradition why God chose which tribes to stand on Mt. Ebal & Mt. Gerizim?
ב"ה Shalom Many commentators(Ibn Ezra, Chizkuni, Rabenu Bechaya and others to Devarim 27:12) explain that the tribes that were born of the actual wives of Ya'akov, Rachel and Leah were chosen to be on Har Gerizim for the blessing while the tribes born from the handmaids Bihah and Zilpah were chosen to be on Har Eival. However, since there are only 4 children born from Bihah and Zilpah and d there was supposed to be an even amount of tribes for each mountain, Reuven the oldest of Leah and Zevulun the youngest of Leah were chosen to be on Har Eval. Zror Hamor adds That Reuven was removen from Har Gerizim to Har Eval because of what happened with him and Bilha. Shimom and Levy were chosen for the Beracha because Hashem didn't want to give them more judgement than alreadu given by Ya'kov in Parshat Vayechi. Yehuda represents royalty therefore he was chosen for Har Gerizim and Yissachar represents the torah so therefore he was chosen for Har Gerizim. Yosef and Binyamin were chosen for the Bracha because Mishkan Shilo and Yerushalayim were in their portion of Eretz Yisrael. Kol Tuv.