Why is it that in Numbers 32:1–32 Moses only commands Reuben and Gad to cross the Jordan River, but then in Joshua 1:12–15 it has Joshua talking about how Moses commanded Manasseh to cross over as well? Some think these different commands are a result of the Yahwist, or Elohist communities of the Documentary Hypothesis.
See Meshech Chochma commentary #43 on Bemidbar (Numbers 32:33). It is a long, so I don't want to copy or translate everything.
The gist of it is that part of the tribe of Menashe afterwards decided to join the other two tribes. Prior to giving the them the land, Moses bound them by the same conditions as Reuven and Gad. This is a Talmudic principle that states that a "vow" or "promise" made based upon a future event is binding once that event occurs.
The example he gives is in Talmud Kiddushin that states that if someone says, "You are betrothed to me once it begins raining." Once it rains, the woman is betrothed.