Cowser, to me, was clearly mentally struggling. You could see it on his face, in his body language, in his movements on the field. I fully supported sending him back down at the time.
I personally don't see the same thing with Holliday. Maybe a little hesitation in the field, but that also could be learning a new position. But he seems composed, you see him in the dugout and he doesn't seem down on himself. He celebrates with his teammates when good things happen. He's generally not up there fishing for bad pitches. I don't get the impression that he's in over his head at all from a mental standpoint.
I'd say it's a combination of around 40% not quite ready, 40% just a badly-timed slump, and 20% luck. He had some really tough calls go against him early on that made his ABs a lot more difficult.
End of the day I don't really care what they do. I wasn't clamoring for him to come up and I'm not going to be upset if he goes back down. But I see no evidence that he needs some sort of reset.