yes it's a bit problematic that the event is fired globally by the root frame and cannot be related to the view where the animation was applied on ...
... but there's a simple solution - do not subscribe in the constructor but rather in the OnNavigatedTo-Event of the View so your event handler gets attached only and always when your user actually navigates to the page. As soon as the event got raised, unsubscribe the event handler from the event so the even't won't get fired by the same event raised from other views later.
Subscribe like that:
rootFrame.PageTransitionCompleted += PageTransitionCompletedHandler;
You can unsubscribe as soon as the PageTransitionCompletedHandler gets called and you can also add an additional check if CurrentSource.OriginalString contains the file name of your current view:
sender, EventArgs e)
rootFrame.PageTransitionCompleted -= PageTransitionCompletedHandler;
// Do your stuff here
So that's pretty solid. This solution didn't cause any problems for me and should do it.