I don't necessarily think he had to have given Kid Boo trouble. If he held-out long enough compared to West and North, while also showing some interesting techniques during the battle, I believe that alone could've prompted Kid Boo to absorb him. We know South Kaioshin had to be much different from the others because Kaioshin even seemed surprised ("somehow absorbed") that he was absorbed.
It being random is something I originally subscribed to, but that seems too simple. 2 of the 5 Kaioshin were killed, the weakest was maimed, the 2 strongest were absorbed. Some logic has to go into that taking place.