The odd ones out
אלה יעמדו לברך את-העם על-הר גרזים בעברכם את-הירדן שמעון ולוי ויהודה ויששכר ויוסף ובנימן: ואלה יעמדו על-הקללה בהר עיבל ראובן גד ואשר וזבלן דן ונפתלי
These shall stand on Mount Gerizim to bless the people, as you pass over the Jordan River: Shimon, Levi, Yehudah, Yissachar, Yosef, and Binyamin. And these shall stand on Mount Eival for the curse: Reuven, Gad, Asher, Zevulun, Dan, and Naftali
One of the commandments the Jewish people were instructed to fulfill as they entered the land of Israel is known as the Blessings and the Curses. The twelve tribes were to divide in two; half would pronounce blessings to the people for those that keep the Torah, and half would pronounce curses for those that didn’t. The Torah tells us who is to stand where: Shimon, Levi, Yehudah, Yissachar, Yosef, and Binyamin are to stand on Mount Gerizim and pronounce the blessings. If we look closely, we’ll see that they’re all children of Rochel and Leah, Yaakov’s primary wives. Those to stand on Mount Eival and pronounce the curses were Reuven, Gad, Asher, Zevulun, Dan, and Naftali. The four children of Bilhah and Zilpah, Yaakov’s other two wives, are in this list. However, Reuven and Zevulun are the children of Leah. Why are they singled out from their brothers on Mount Gereizim, and told to stand on Mount Eival?
Perhaps the Torah is addressing the famous episode of Reuven’s “sin” with Bilhah. After Yaakov’s favorite wife Rochel passed away, he moved his bed into her maidservant Bilhah’s tent. The verse then says that Reuven slept with Bilhah. This would seemingly have been one of the worst crimes imaginable against his father, who was married to Bilhah. Reuven should have been excommunicated, banished from the family. However, the verse continues to say that the tribes of Israel remained twelve. Chazal saw from here that the verse isn’t to be read literally. What it really means is Reuven disturbed his father’s sleeping arrangement, by moving his father’s bed out of Bilhah’s tent. He felt his mother Leah should have been given precedence before Bilhah.
One could say that the verse is merely expressing the severity of his crime in harsh words. Yet, it’s not that Reuven actually committed adultery with his father’s wife. However, the verse is pretty explicit. This alternate reading appears to be a stretch, and there’s definitely room for someone to doubt Chazal’s interpretation. To prevent any suspicion against Reuven and his innocence, perhaps it is for this reason that the tribe of Reuven was placed on Mount Eival. One of the curses which everyone there pronounced was: “cursed is he who sleeps with his father’s wife”. Hashem wanted to make it clear that Reuven was innocent of this crime, and therefore worthy to pronounce such a curse to those that deserve it. However, Zevulun’s placement on Mount Eival still needs explanation.
The tribe of Zevulun had an arrangement with the tribe of Yissachar. Zevulun would go out to sea on business, and Yissachar would stay behind and learn Torah. Zevulun would then return home and split the profits with Yissachar, on condition that Yissachar split their spiritual reward with Zevulun. Perhaps because Zevulun supported Yissachar’s Torah learning, they were placed on Mount Eival. What’s the connection?
One of the curses pronounced was: “cursed is he who does not uphold the Torah”. We are taught that even someone who fulfills the entire Torah, if they had the means to support Torah and didn’t, they are included in this curse. As such, perhaps someone may suspect Zevulun of not supporting their brother Yissachar to their fullest. Maybe they were lazy in their business voyages, and didn’t accrue as much as they could have. Or they didn’t fairly share all the wealth they had amassed. To avoid such suspicions, Hashem placed them on Mount Eival. This would show that they too were worthy to pronounce such a curse for those who fail to uphold the Torah.
 Based on Ta’amah D’Krah to Deuteronomy 27:12,13, by Rav Chaim Kanievsky shlita
 Deuteronomy loc. cit.
 Interesting to note is the Bahag (and the Geonim who followed in his path) in his introduction to Halachos Gedolos counts this as one of the 613 mitzvos (Parshiyos § 64). The Rambam in Sefer HaMitzvos Shoresh § 3 argues on this and says that this was a mitzvah for a one-time event in history. Since it wasn’t for all time, it’s not worthy to be included in the 613 mitzvos. He was preceded by the Ibn Ezra in his Yesod Moreh Sha’ar 2, who also asked this on the Geonim. The Ramban ad. loc., although he agrees with the Rambam not to count it, tries to defend the Bahag. He suggests that since the content of the mitzvah applies for all times, namely accepting to keep the Torah on condition to receive the blessings and the curses, even though the action of the mitzvah was a one-time event, is still worthy of being counted in the 613. However, see Rav Yerucham Fischel Perla’s commentary to Rav Saadiah Gaon’s Sefer HaMitzvos LeRasag Parshiyos § 57, who explains the Geonim’s intent is to count the mitzvah of arvus, the Jewish people’s responsibility for one another (as derived in Sotah 37b)
 Genesis 35:22
 Shabbos 55b
 In fact, some commentaries explain the verse literally, such as Bechor Shor, Radak, Ralbag, and Paneach Raza (in two explanations) ad. loc.
 Deuteronomy 27:20
 Bereishis Rabbah 72:5, 99:9; Vayikra Rabbah 25:2. See Deuteronomy 33:18
 Deuteronomy 27:26
 Yerushalmi Sotah 7:4