I think I know why. Galavantula had unnurve as an ability. So when Mandibuzz went below half health first, it didn't eat its berry. But when Galavantula fainted, there was no longer any unnurveness, so it ate its berry.
But if Mandibuzz wasn't under half health, then, maybe eating berries early is just a side affect of this type of thing.
I hope I make sense.