SQL Windowing Functions: Are the results ordered?
Without a presentation ORDER BY clause, standard behavior returns a set in no particular order.

I think Itzik Ben-Gan explains it best and even without one of his books, you can find the explanation in this article. About 12 paragraphs down, next to the first code sample, the text describes how the ORDER BY in the window function serves a logical function to describe the set, and how that is completely separate from a presentation ORDER BY clause.

A related question appears here
Is SELECT ROW_NUMBER() guaranteed to return results sorted by the generated row numbers?