Generally speaking, I think what got the WLW is that something happened outside of his/Yatwer's "infinite vision". It's a pretty blatant demonstration of "the Gods are blind to what they are blind to".
If Kellhus came back, its hard to assume that he didn't always plan on it. Or, maybe that nuke damaged the Ordeal in such a way that suddenly Kellhus figured he better make sure plan B (ie, whatever is going on with Empire and Zeum) is going as planned.
Contingencies for running out of sranc seem to be planned for, so I don't think the dispersion of the Horde-Meat-Cattle was the trigger for Kellhus coming back. Maybe, though, this happened sooner than what he expected, but to me it doesn't seem a huge stretch that kellhus figured a decisive victory at Dagliash would destroy most of the sranc one way or another.
If not for the nuke, the sranc would have just kepts pouring through the mountain passes, dying in the millions. Incidentally, the tekne-nuke might have ended up saving more of the sorcerers than it destroyed. The schools seemed to be pretty hard pressed on the mountains, and they were starting to lose some members. Assuming that the Swayali got away and were able to warn/protect some of the schools, the whole thing might have really been a net gain. We have already been told the Ordeal is really just a meat shield for the Schools. It'll just be a bit more Meat than Shied as they cross Agongorea.