In a thread like this, I think it is. “Because the author said so” is such an obvious non-answer it should be dismissed out of hand as being counter to discussion. I know Stephen King made it up, you know it, the thread starter knows it, so why bother bringing it up as an answer?
Because it is an answer; if something is deliberately unexplained in a book or movie, it is very often the answer.
But he didn’t say it was deliberatly unexplained, which is a fair answer – I actually give it above – he said it was cause King said so. Which is a complete non-answer as far as threads like these go.
The distinction eludes me, but whatever.
le sigh.
“It’s unexplained” = it’s unexplained
“It’s only a book” = it could have an explanation in the book but you don’t need to know it because it’s only fiction
One answers the OP’s question. The other doesn’t.
Funny, I didn’t read the comment as ‘it’s only fiction’ - I read it as there wouldn’t be much of a story if evil wasn’t involved, it’s not really relevant why and how that situation came about.
Y(incredibly serious)MMV, obviously.
Anything could have an explanation; anything at all.
Except, seriously, it’s Stephen Fucking King. Not many people know this, but he got the “King” part of his name because he’s the “King” of just pulling shit out of his, um, imagination for the sole purpose of keeping the plot rolling.
When you’re talking about the creator of a fictional world–especially when fantasy and the supernatural evolved–“because the author says so” is often the only possible answer. Times a gajillion when it’s King.
Sometimes the author pulls some background out of his, um, imagination, by way of setup or justification. But sometimes he doesn’t. Times a gajillion when it’s King.
Even if you trace the “reason” asked for in the OP back, eventually you reach a point where the ONLY answer left is “because King said so.”
Well, does it help any that “Zombie == bad, flesh-eating horror” is a fantasy cliche and King was just being unoriginal?
Ahem.
:dubious:
There is the factual, actual answer that really exists, despite the cries of those in this thread who feel it doesn’t have to. 
And to take it another level, the sour ground of the Pet Sematary/Micmac burial ground is more a metaphor for the “soil of men’s hearts” which is “stonier”, as the ghost Victor Pascow repeats a few times in the book. Louis Creed has planted a family and he’s not sure if he’s truly connected to it, if his heart is not perhaps just a little too hard to fully integrate with them. Not until he begins to lose them does he learn that his heart is indeed tougher, capable of more terrible things, but he also learns, too late, how full of love it is.
The story is about a normal man who must face the ugliest parts of his own soul, which have been exposed by a horrible tragedy, and it’s also about the human ability, or lack thereof, to come to terms with mortality. All the stuff about Indians and zombies and so on is there to represent aspects of this long human struggle with the finality of death (which symbolism hardly new, of course).
And if anybody wants to know anything about the symbolism and theme of Tommyknockers, I’m definitely your man.
Oh good grief. It’s a King story. He does this all the time. I should know, I’ve read 99% of his works. No explanations as to why. It just is. In his universe. If recognizing that is “thread-shitting”, then truly the fight against ignorance is lost.
Oh, he was just kidding around-lighten up, wouldya?
Besides, King has said on occassion that he finds it more interesting to make things happen “just because.” That sometimes explanations spoil the story.
Disclaimer-I haven’t read the book
Could it be that as the Micmac were wiped out by settlers (deliberately, and through accidental disease, and just by being displaced by the sheer volume of honkeys) their sacred ground became poisoned by their rage and despair?
Or, could it be that all the centuries of burial have just altered the soil ph? The ground is sour now, but some alkaline chemicals will fix it right up.
Or, could it be that after the destruction of the One Ring, some dark spirits were still able to cling to this world by the barest of margins. After millenia of waiting, they at last have the strength to reassert their presence in burial grounds, like the barrows of old.
I believe there is a definite undercurrent of this in the book, though the above thing about cannibalism is true. While there are spirits in the woods beyond the Pet Sematary (so called because the sign for the place was written by a child; an important point of the story is that there is a ‘good’ Sematary in front of the old, bad one, and it’s a place where many children come comfortably to terms with natural death for the first time), why they walk the earth is one of those Kingian mysteries others have alluded to in this thread. There are strange things in his world because his world is a strange place where monsters are real and magic works. God, too, exists in this world, though God is curiously absent in Pet Sematary.
I’m rambling. Perhaps I should stick this in an MMP. 
Anyhow, my point is that while some of King’s critters do just exist of their own accord, many, many of them - in Cujo, in The Shining, in Firestarter more literally - are absolutely the creation of human minds/emotions/fears/sins. Very often the latter, as King is full of tremendously Judeo-Christian moral lessons. Cheat on your husband, and your son will die of dehydration while you’re trapped in a car by a rabid dog. Drink and wallow too much in your own self-pity, and you’ll kill your own family with an axe.
I’ve drifted again. The point is, sometimes the monsters are more or less born directly from the evils of humanity, and I think there’s more than a hint in Pet Sematary (with the ever-recurring image of The Road and the speeding, crushing machine Truck ripping up the landscape and taking cats and babies with it) that the whole evil Indian spirit business rose out of the rage and misery and terror stemming from the American Indian near-genocide.
It’s been 10 years or so since I read the book, so I may be wrong…but I got the impression that the dead came back evil because the corpse was inhabited by a malevolent spirit and that the same thing had happened even when the Indians had used the place.
Second part yes, and I want to address something about that in a moment, first part…maybe. There’s a hint of that, but there’s also a suggestion that death and return simply ‘sours’ the soul beyond all recognition. One point I think is very important is that it’s not the evil that comes from the ground that matters, in the story: it’s that it is evil to bury things there. It’s the sin of the person trying to thwart death that King is focused on.
As for the Indians: it is clear in the book that they bear at least part of the ‘responsibility’ for the souring of the burial ground where victims of famine-based cannibalism were laid to rest. But it’s possible this was caused by the evil in the first place, or just made what already existed more powerful, as the extinction of the Micmac would also do.
The animals weren’t so much evil as they were extremely irritated. You’d be pissed too if worms constantly burrowed through your flesh.
It’s been a while since I read the book as well, but it seems to me (and I doubt this is supported at all by the text; it’s really just my impression) that intelligence mattered. That is, the dog that came back came back as something soulless and empty, but not necessarily evil. It was just more of a lump. The cat that came back was mean, but again, it didn’t seem evilly so - it just seemed to do the things that cats do, with perhaps more malice, but I don’t remember it setting out to do harm. However, when people were buried up there, they came back vastly worse, and my guess is because they had more intelligence when alive than your average household pet. You could call it my “residual intelligence” theory - the more intelligence the creature has to start with, the more chance they’ll come back with the intent to do evil.
Perhaps King was just saying that animals can’t really be evil - that’s a property unique to people. I don’t recall Cujo being evil either; he’s just rabid. But it’s been long enough that I may be way off there.