While scrolling forward through your script in Screenwriter 6 on Windows, occasionally the view will skip backwards by a page (or several pages).
If you look at this picture showing the lower right corner of the Windows Screenwriter editing window:
You can see the bottom scroll arrow for the document window (looks like the letter 'V'). You were most likely scrolling down using that (right)?
Well, immediately below that icon happens to be the icon that looks like two upward-pointing triangles. That icon does the "go one page back" function.. You have occasionally missed the scroll down icon by a few pixels (easy to do) and accidentally hit the Go One Page Back icon.
Solution: make sure the tip of the arrow of your mouse cursor is in the middle of the scroll-down icon. OR, use the down arrow key to scroll down through the document.