Calling getForLabels(Page.class, labels) for example, returns a List<Page> that contains all ContentEntityObject s matching the labels passed in, and not just Pages.

For example, if you created a Page and a BlogPost with a label1, and called

List<Page> myList = labelManager.getForLabels(Page.class, label1);

You get back a list with both your Page and your BlogPost, so when you try to fetch the BlogPost result as a Page, you get a ClassCastException.