Kapler scripts a lot of bullpen stuff, and they react to situation and data. But I think in this case they expected Neshek to get three outs and then decided, in the moment, that he wasn't ready to pitch, having just come off the DL.
They also could have stuck with Hunter, but I don't think anyone wanted that. Agreed it was curious how they timed Leiter's use, especially since they knew Rios and Dominguez were unavailable (still not sure why, exactly, on Seranthony, but can't blame them for finally being conservative about him). I think at that point in the game he wasn't managing for extra innings. tbh I was stunned he had a batter left.
I'm sure in the post-game interviews Kapler said that they knew Pivetta was available today, as far-fetched as that would have seen.