I feel overtiredness will always be a reason. It not the only reason, but she is a fiery little girl who will not ever back down. She sleeps in the same room with her two siblings (6&2.5) so it keeps them from going to sleep peacefully too. (baby sleeps in with us)
It's so hard because we both try the technique of making the point of the importance of sleep, but we equallly don't want to be bossing or shouting but she gets us to the edge some times. And she gets herself to the point of no return and can't calm herself, or let us calm her.
I try to think if anything has happened, changed, poorly, or anything but it blows hot and cold most weeks.
She's at school and does get tired but it comes across more of a power thing sometimes to see how we will react? Tiredness is a huge factor but the not listening to when being calmed down or the fact she thinks it's acceptable to shout back is beyond us. We've never smacked. It she gets us to the point of raising our tone as she makes us that furious.
We treat them all the same (within reason of age at time but even the two year old goes to bed nicely!) she does go nicely some nights, weeks even but the weeks that are bad, she is tiring. And leaves us thinking we're failing!