It's usually the same culprits! If we were playing Family Feud, these would be the winning answers:
1) Hungry/hangry
2) Overtired
3) Has to poop
4) Is possessed and no one knows why
5) Not sure what's causing the naughtiness, but pretty sure it's because I did something wrong as a parent.
If your LO is naughty, what are the usual causes?