I had been thinking about a BIG spell. Cast by someone extremely power and at least somewhat annoyed. After all, it's affecting the whole world. Or else a conspiracy of like-minded people (who must recruit to get the right mind set.)
And then I pondered a case where spells interacted.
Would have enormous impact on your rules of magic, of course, even if they were big spells. But if you wanted something like Neverwhere, without the disappearance being mysterious and inexplicable -- imagine a time where one wizard wanted all these riffraff out of the way because of the way beggars were fouling the city or panhandling everywhere, or menacing or even assaulting people, and another wanted to protect the poor unfortunates from the hostility and abuse -- and both of them at once (when hostilities had reached a peak) cast a spell to hide them from the ordinary citizens. It would take only a little tweaking with magical times to make them do it at the same time -- at the new moon, perhaps.
Would probably also feature as an object lesson on the dangers of not ensuring your magic doesn't interact, at least among the hidden people. 0:)