I reloaded, had it happen again, reloaded again and had it happen again. Same 100 peasant unit in the 3rd, 9 unit gallowglass in the 2nd.

I noticed that you can set a reinforcement point farther than you can order a unit to walk to. What I did was withdrew one of my units, set up the reinforcement point as close as possible to the peasant units and then called for reinforcements. After or immediately before arriving at the reinforcement point, the battle ended. I guess the reinforcement unit came close enough to the peasant unit that the AI decided to withdraw the peasant unit.