I'm guessing you have Click and Type enabled (Print Layout view only).
Depending on where your cursor is between the margins, you'll see lines
representing Left, Center, or Right paragraph alignment. If you double-click
in that spot, you'll get a new paragraph with the designated alignment.
You'll also very likely get a forest of empty paragraphs, spaces, and tab
characters between where you clicked and where the existing text leaves off.
If this horrifies you as much as it does me (though I can see how it would
sometimes be useful), you can disable Click and Type on the Edit tab of
Tools | Options.