It could be from:
-Loading the game up an extreme amount of times
-I think one had an SD card, and some can't handle AC that well
-Quitting without giving the game a few minutes to finish getting everything set up (I don't think that if you save this happens, because it... Well... Saves everything XD)
-Quitting the game as soon as they enter the campsite
-Quitting the game during a loading screen
-Many more that will take up too much space XP
The problem with this is though, the game does not save at all on the new alt. Closing a game off when it's loading the town is one thing(such as on your already created characters), as the game does a mini-save when you're logging in.(Which is the reason the reset trick works). There's absolutely no way I can buy resetting the moment you enter the tent is at risk, because the game already loaded if it's displaying the animal. And as for the loading up multiple times, if the game save could become corrupt like that, wouldn't people in other games be complaining that their save files were also corrupt? For example, people can soft reset in Pokemon thousands of times, and there is no ill-effect. I can't see why the 3DS cartridge would be so drastically different that loading several times could damage it. The only way I can see it possibly corrupting the save is if the person accidentally loads the mayor, and shuts it off when Isabelle is loading up the town when they realize the mistake. ((As mentioned before, the game is doing a save right then.))
In general, my issue is if the reset trick
did corrupt games, why are there only two to three people that are talking about it? I've never seen anything mentioned on it previously, and with the amount of people doing it, there should definitely be a larger sample size of complaints. I think there is probably something else going on. The ones who had their game save data corrupt didn't think of mentioning as they probably didn't believe it was related. (such as dropping the 3DS during a reset attempt or loading accidentally on Isabelle and shutting it off during her save)
As I mentioned previously, I've done the reset trick between my friend and me at least 3,000 times.(For me alone, I have 901 loads on my first game and 619 hours, second game I have 395 loads and 267 hours. At least 80% of those loads are from the reset trick.) We both have our towns on SD cards. We both use different resetting methods.(She does the home and x off the game way. I use power button then home.) We both have had our games since June 9th. If resetting really could harm the game, we both would be the prime candidates for it to occur with. I typically reset the moment I enter the tent unless I'm at eight villagers and looking around to see if someone set up a plot. If I really feared that it would corrupt my game, I would stop. However, I cannot see any plausible explanation for it being the source of the corruption.