There is probably paragraph formatting such as space before, space after, keep lines together, etc. affecting the behavior of the text. This may be on the text you're pasting in, or already in your Word document and the text you paste is inheriting this formatting.

If Graham's button above doesn't solve the mystery for you, select the paragraphs then click the paragraph formatting dialog box launcher. That is the tiny button in the lower right corner of the Paragraph group on the Home ribbon.

Once in that dialog box with the problem paragraph(s) selected, you can remove all space before and space after and set line spacing as you want it. Also, notice there are two pages inside the dialog box. Click the Line and Page Breaks tab to make sure that Keep with Next, Keep Lines Together, or Page Break Before are not checked. Any one or a combination of those may explain why it's jumping to the next page.

Note that you can also set the formatting you choose as the default for the document you're in plus future documents created from the same template. I would do that if you also have undesired spacing when you are just typing in Word.