OnDisappearing event fired in the first page of the NavigationPage stack.

Hi,
I'm pretty new on xamarin form and I would like to understand why
if I initialize my first page in the NaviagationPage also the event OnDisappearing is fired.
That's my code in the app.cs file:
public App ()
{
InitializeComponent();
MainPage = new NavigationPage(new Page1());
}

and at the startup of the app the event OnDisappearing (I ovveride the method) is fired in the Page1 but the Page1 is visible properly.

Thanks