Isso nunca é explicado
Curiosamente, um rascunho inicial do capítulo é menos definitivo sobre o assunto, levantando (embora não confirmando) a possibilidade de que a escuridão estava se separando de acordo com o cronograma de Sauron:
But it was no orc-chief or brigand that led the assault on Gondor. Who knows whether his Master himself had set a date to the darkness, designing the fall of the City for that very hour and needing light for the hunting of those that fled, or fortune had betrayed him and the world turned against him? None can tell.
History of Middle-earth VIII The War of the Ring Part 3: "Minas Tirith" Chapter IX: "The Battle of the Pelennor Fields"
Analisando todos os meus livros, essa é a coisa mais próxima de uma explicação que posso encontrar. Em Companheiro do Leitor , Wayne Hammond e Christina Scull observam a possível conexão com Manwë, notada por Jonas em um comentário sobre a questão:
Eles vão notar uma semelhança com outros dois exemplos de vento soprando o Mal, que também são normalmente atribuídos a Manwë e os Valar (embora sem a confirmação de Tolkien):The wind from the south is blowing away the darkness that flowed out of Mordor, and speeding the Haradrim ships captured by Aragorn up the Anduin. Some readers have speculated that this is due to the unseen intervention of Manwë, chief of the Valar, whose province is the winds and breezes and regions of the air
The Lord of the Rings: A Reader's Companion Book 5 Chapter 6: "The Battle of the Pelennor Fields"
[A]s the Captains gazed south to the Land of Mordor, it seemed to them that, black against the pall of cloud, there rose a huge shape of shadow, impenetrable, lightning-crowned, filling all the sky. Enormous it reared above the world, and stretched out towards them a vast threatening hand, terrible but impotent: for even as it leaned over them, a great wind took it, and it was all blown away, and passed
Return of the King Book VI Chapter 4: "The Field of Cormallen"
To the dismay of those that stood by, about the body of Saruman a grey mist gathered, and rising slowly to a great height like smoke from a fire, as a pale shrouded figure it loomed over the Hill. For a moment it wavered, looking to the West; but out of the West came a cold wind, and it bent away, and with a sigh dissolved into nothing.
Return of the King Book VI Chapter 4: "The Field of Cormallen"