I believe it's much the same as the "insulting the enemy" status they have when you start deployment, in that it has no numerical effect (stats and so on), only describes their animations. But I have noticed that even during a battle some units sometimes get the "celebrating" status. This is usually with a unit that has just utterly destroyed an enemy unit, or when all enemies are routed (and you continue to take prisoners), and it only happens with units that have no standing orders at the moment.
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