Groups of units get a stack bonus that maxes at eight. This makes eight-man formations fairly common. Any special bonuses apply to the entire stack.
Stacks without a leader are forced to auto-attack when in contact with units from non-allied capitals. With a Commander the fight can be directed, or the group can choose not to initiate a fight.Erf-b1-p040a
It could also be that it was simply more efficient for the stacks to be separated, as their move stat was so different that, rather than slow down the fastest units to have them all arrive at the same time.