Not every ad hominem is a logical fallacy, nor is it necessarily a personal attack. In this instance, what the defender of the PSR proposes is that the skeptic's own presuppositions entail adherence to the PSR. Have you ever been involved in a discussion like this?:
Proponent: God exists because X, Y, and Z.
Opponent: Why should I believe Z?
Proponent: Because Z is based on the PSR.
Opponent: Why should I believe in the PSR?
Notice how bizarre that last question sounds if you step back for a moment to think about it. If it's not the case that every existing thing has an explanation of its existence (PSR), then why should the proponent of the PSR be required to give a reason in defense of the PSR? If there are so many exceptions to the PSR, then it seems arbitrary for the opponent to demand an explanation in this case but not in others.
It seems to me that the PSR is a first principle of rational inquiry, much like the assumption that an external world exists. Without such principles, all rational inquiry ceases. Moreover, if we assume that the PSR is false, that gives rise to all kinds of paradoxes and absurdities. The proponent of the PSR should also not overlook the strength of arguing from intuition. How many of us, Richard Taylor points out, upon walking by a glowing translucent ball in the middle of the forest, would conclude that the ball had no explanation whatsoever? My guess is that even the diehard skeptic would not hesitate to conclude that it has an explanation, whatever that explanation might be.