Our girl recently decided to start sleeping in her own room. It was triggered when she caught a cold, got exiled from the main bedroom to avoid getting her brother sick, but then chose to stay in her own spot.
Unless it is cyclical or seasonal, change tends to feel quite sudden.
In retrospect, one might be able to see it building up. Like water turning into steam, you know it’s getting hot, but it still seems to jump between states in a blink of an eye.
Growing up is weird, even for the parents.