The S"A 125:1 (YD) states that a non-Jew pouring non-Mevushal wine makes (at least) that wine unfit, but other than citing earlier authorities (maybe, I can't figure that part out) he doesn't give a reason, and I must say, I can't figure out why.
It isn't because of idolatry as there is no physical contact for a stirring libation, extra pouring for a pouring libation or shichshuch for a shaking libation, and the rema, in 123 states that simply touching a container of even open wine (non-mevushal) is no problem.
In fact, this was not a concern in Masechet Avodah Zarah 71a. In the mishna there, a non-Jew lifts up barrels of a Jew's wine and pours wine out to make a purchase. The only question there is one of ownership (kinyan hagba'ah). If the ownership is not effected, the wine in the barrel doesn't become unfit. Where does this additional measure of forbidden-ness come from?