In a word processing document (Pages default), body text is designed to flow automatically from one page to the next. If you increase the line spacing of selected text on page 1, it will push the last few lines of that selection onto the following page. Works exactly as it is designed. Your only means of controlling this is to 1) enclose the affected text in a text box, 2) choose a Layout style template, or 3) change the word processing document type to Layout, which you do by deselecting Document Body in the Document panel. Only do this with a new blank document because done on an existing word processing document removes its contents. Not a good feeling if you are not using Time Machine or another means of backing up your documents.
In a Layout mode document, similar to Keynote, there isn't body text to automatically flow from page to page. Instead, all text is constrained within text boxes that you insert and control the flow of text by choosing whether you will have standalone text boxes, or by linking those text boxes on the same or different page.
More on this in the Pages for Mac User Guide.