MS Excel techies, HELP! Teach me how to do something with MicroSoft Excel

I almost always keep 6 or 7 spreadsheets open so they are stacked at the bottom of my PC's screen in the order that I opened the spreadsheets.

When I put the pointer over the stacked spreadsheets in the tray they will all popup horizontally across the bottom of the screen in miniature form so that I can move my pointer over the one I want then click on it to expand it to fill my entire screen.

Occasionally one of the spreadsheets in the "stack" at the bottom of the screen will inexplicably move to a different place in the order of the stack.

How do I move it back to its previous place in the cascade without closing out all of the spreadsheets and then reopening them in the order I want them to appear in the stack?

Thanks.

PS: My operating system is Windows 8.1.