Why does the Torah reverse the order from "B'nei Reuven u'Benei Gad" in the previous Pasuk to "B'nei Gad u'Benei Reuven" here?
Ramban and Moshav Zekenim: Because on the one hand, Reuven was the oldest of the tribes, whilst on the other, Gad was the one to put forward the proposition and subsequently became the spokesman for the tribes involved, as well as being the strongest. 1
Oznayim la'Torah: Because, basically, the Torah always gives precedence to Reuven, who was the B'chor and the son of one of Ya'akov's main wives - Le'ah, whereas Gad, was the son of the maidservant - Zilpah. And the Pasuk mentions Gad first for the remainder of the Parshah, because they were the ones who were most interested in inheriting in Eiver ha'Yarden and who subsequently pushed for it, and Reuven only followed suite. 2
See Ba?al ha?Turim and Oznayim la'Torah, who cites many examples from Seifer Yehoshua.
See Oznayim la'Torah, who gives three reasons as to why Reuven were not as keen as Gad to receive their portion in Eiver ha'Yarden: 1. Because of the dnager posed by the neighboring nations of whom the mighty Gad ("ke'Lavi Shachein Vetaraf Zero'a Af Kodkod" - Ve'Zos ha'Berachah Devarim, 33:20) was not afraid; 2. Because Gad's flock's were far in excess of that of Reuven (See Oznayim la'Torah) and their need for good pasture was more acute than that of Reuven: 3. Because, because, by receiving their portion in Eiver ha'Yarden before the rest of the tribes, they would not receive a double portion in Eretz Yisrael - See Oznayim la'Torah.
