I have only read the first book in S. M. Stirling's Emberverse series, Dies the Fire, but found myself frustrated that the underlying cause of “The Change” was not revealed and thus did not continue with the series. Is the cause ever revealed? If so, in which book?
In The Sword of the Lady Juniper and Mike's son, Rudi, meets "someone" when he gets given a magic sword and all is explained, kind of, basically the "powers that be" stole Nantucket Island and sent it back in time (as detailed in Island in the Sea of Time) to change the outcome of humanity's technological progress from "absolutely going to destroy all life on Earth" to "might destroy all life on Earth" because life on Earth is important because... reasons. The whole thing is about two thirds of Chapter 21 and not much of it makes sense without getting there the hard way anyway so I'm not going to quote you from it; the gist is that ideas eventually take on a mind of their own, to the extent that they outlive their creators, and they seek to reinforce themselves, or at least to guarantee their continued existence.
This answer is really only half-satisfying, but here is what I've found so far:
An enormous electrical storm of unknown properties encompasses the island of Nantucket and transports it back to the bronze age. The resultant time shock causes everyone on Earth to suffer an intense migraine at the exact same moment. The far more important consequence is that any device run off of electricity, gunpowder, explosives, internal combustion or steam power ceases to function. Permanently. In a single instant, humanity has been ''literally'' bombed back to the Stone Age.
Someone on the Emberverse Wikia seems to feel that:
The effect appears to be caused by an alien intelligence in an effort to minimize the ability of a competing civilization to wage war in their universe. These aliens apparently have the ability to affect matter and energy at the quantum level and have access to multiple universes and timelines. There is no evidence that the Change is reversible.
I haven't read the books, and since there are no cited sources to be found there, I can't vouch for any of that information. So I can't tell you when this information is revealed. But a Google search of medium-depth reveals that there isn't a whole lot of verifiable information out there on what this is, which makes me think that it's never really explained.
Besides - without technology, how would people on Earth be able to research the event and figure out what happened?
Part of the explanation would spoil stuff that comes later in the series, particularly the 2nd set of stories involving the next generation of characters. Suffice it to say that Stirling goes into more detail in "The Protector's War" as to the nature of the changes involved, in which Ken Larrsen metaphorically bangs his head on the wall as he discovers that what has occurred is specific to only certain things, ensuring that humanity can't have its 'toys' any longer.
Love the series and how it evolves from straight "post apocalypse" yarn to a more fantasy based series as it moves along.
It's implied that aliens may have been involved, but in the latest book it is confirmed that an outside force stopped technology from working so that humans would not destroy themselves and would have much more respect for their technology when and if they get it back.
The story is seemingly about the "Old Gods" returning to our universe to protect mankind from destroying ourselves. It kind of postulates that there was once a time in our history where supernatural beings were a part of our reality, the Gods intervened from time to time, etc. This period ended and we developed modern technology and science, but we so abused that knowledge in many ways that we threatened the continued existence of life on earth, so those old powers stepped back in and caused the Change. There is more to it than that of course, there are powers from outside of our universe that are seeking to enslave mankind and must be battled to prevent their success. Of course the form those external powers take in our reality depends a lot on your point of view, so the Christian Monk in the story has a vision of Mary Magdalene, the Wiccans encounter their God and Goddess, those of Norse heritage meet Odin etc. The reference to aliens comes from an explanation for the Change made by one of the characters where he says we have no idea what caused it and its completely fruitless to try to figure it out given the circumstances, he then makes a casual reference to "Alien Space-Bats", but this is not the real cause of the Change just a spurious statement to say it was caused by something beyond our ken. This is my all time favourite SF/Fantasy series, and I highly recommend everyone try it to see if it suits their needs. Its a great tale with a lot of great characters in it.
Eventually what emerges is that there is a battle among the gods. In order to prevent one side, which wants to completely reorder man as a sterile, perfect creation (which requires first annihilating us using our own technology) the universe was subtly altered so that this technology was no longer able to work. Thus the battle between one set of gods and the other is carried out on earth through intermediaries using fire and sword instead of nuclear weapons and biological warfare. This is actually an outstanding series and I highly recommend it.