I can't explain WHY it happens. It is normally not an intentional change in the Layout and adjusting it in the Layout does not fix it or prevent it. Maybe we slipped on the mouse? Maybe the moon is in the wrong position? Maybe it's Gremlins?
Move your mouse DOWN the "empty" part of the page (the part where the red question mark is showing above), until you see a "curser tool" that looks like two arrows (up and down) with a vertical line between. This may be so far down the page that you have to use the scroll bar on the right to scroll down far enough to find it.
Once you find the line and the "curser tool", click your mouse and hold it down, while you drag up. By doing this, you have control to drag the tabs back where you want them!
PS: This can also happen in the Detail Views of Groups, Companies, and Opportunities, so you can be ready if it does...