Why did the tribe of Manasseh split?

When the families of Reuben and Gad requested to receive their portions of land east of the Jordan (Numbers ch. 32), one-half of the tribe (the families) of Manasseh (Menasheh) decided to join them and take land east of the Jordan (Deuteronomy ch.3). The other half of Manasseh decided to have their land-portion west of the Jordan together with the majority of the twelve tribes.