It happens when both tamagotchis aren't the same age.Sometimes the nazotchi comes as the mail man to deliver the mail or present to the other tamagotchi.That pretty much all to it.Hope I helped
-hollywood_angel
oh,and is this the tamagotchi thats coming to your tama look like this: Nazotchi
if so,then it's comepletely normal I know from experience with this tama.
Some of the newer tamagotchis don't have the same characters programmed into them as the old tamagotchis. So when the new tamagotchi has a character appear on it's screen that it doesn't recognize, it turns it into a nazotchi on the screen.
The same thing happens if the new tamagotchi has a character that the old tamagotchi doesn't recognize.