First, review
Word Doesn't Know What a Page Is by Daiya Mitchell, MVP.
Word is a text reflow type program. When you add content before other content, the older content moves down.
If you start your pages with a paragraph (heading) that is formatted to have a "page break before," content added before that will be in the page before until a page is filled up, in which case a new page will be created.
See also:
Moving/Reorganizing Pages in Microsoft Word