There's a few elements that can trigger ally/enemy warrior events.

• How many times you've killed X. For example, doing same stage repeatedly killing same Wu officers increases the likelihood that a Wu enemy warrior appears, while helping your allies probably does the same for ally warriors.
• How the battle is going. Since it's so easy to screw up in Nanman Campaign and have bad morale, you can get an ally warrior quite easily in that stage.

There are some other stages, too, where it's relatively easy to get one. I think I once got one in Battle of Bo Wan Po Cao Cao side? Like Nanman campaign, unless you manage to stop the enemy ploys, your morale just continously plummets, and your allied officers die, so getting an ally bodyguard is probably easier in that stage.