I had a lot of slaves stored up in Parabela, and wanted to move them to the Mt Pangeaus mine sites. So I put them in a group with a unit of scout cavalry as escort, but it put the cavalry at the head of the column, and those at the back rebelled. This then required a lot of attention to defeat and recapture the rebels, and get them moving again.
Maybe slaves in a group with a combat unit shouldn't rebel. Would make move them around easier