mpb screen flipping sideways

[oceangirl10]

My mpb late 2011 screen when in Word slides to the left randomly. I'll have two documents open and be working on them when suddenly one slides off the screen to the left. If I try to find it on the File Open Recent, the current word document will slide to the left so the other one takes the screen. I have to close one and reopen it to get them both on the same screen again. It does this randomly on its own without me having my hands near the keyboard.