parshanut torah comment - Why did Moses add part of the tribe of Menashe to join Re'uvean and Gad on the east of the Jordan?

In response to Reuven and Gad's request to settle on the east side of Jordan, Moses includes part of the tribe of Menashe to join them, as it says:

Numbers 32:33:

וַיִּתֵּ֣ן לָהֶ֣ם ׀ מֹשֶׁ֡ה לִבְנֵי־גָד֩ וְלִבְנֵ֨י רְאוּבֵ֜ן וְלַחֲצִ֣י ׀ שֵׁ֣בֶט ׀ מְנַשֶּׁ֣ה בֶן־יוֹסֵ֗ף אֶת־מַמְלֶ֙כֶת֙ סִיחֹן֙ מֶ֣לֶךְ הָֽאֱמֹרִ֔י וְאֶת־מַמְלֶ֔כֶת ע֖וֹג מֶ֣לֶךְ הַבָּשָׁ֑ן הָאָ֗רֶץ לְעָרֶ֙יהָ֙ בִּגְבֻלֹ֔ת עָרֵ֥י הָאָ֖רֶץ סָבִֽיב׃

And Moses gave unto them, even to the children of Gad, and to the children of Reuben, and unto the half-tribe of Manasseh the son of Joseph, the kingdom of Sihon king of the Amorites, and the kingdom of Og king of Bashan, the land, according to the cities thereof with their borders, even the cities of the land round about.

Did any people from the tribe request to be included? If so, why? Or, did Moses force this upon them? Why specifically the tribe of Menashe? How was the division decided (i.e., who should go and who should join the other tribes on the west of Jordan?)