Why does my Visual Studio 2010 default to a horizontal windows split if I quit then reopen it?

I too had exactly this problem running VS2010 with no addins. It appears that everyone else submitting comments and solutions really hasn't understood the problem.

Here is my solution... The problem manifests itself when you close down Visual Studio with the windows in vertical tab groups. Then when you re open the solution FROM THE SOLUTION FILE, the tab groups are now grouped horizontally. (So no, nothing is "saved")

The problem does NOT occur if you open Visual Studio, then open the project from within Visual Studio!

This is actually a feature called Tab Grouping and it can be enabled horizontally or vertically. As far as I can tell, it's the same thing as a window split to enable you to see 2 or more sections of the same file simultaneously. The difference is that Tab Grouping is saved between sessions i.e. if you close and re-open Visual Studio, this setting will be retained!

To disable it:

In your particular case, you have 3 files opened in each group. This means you will have to repeat the procedure 3 times either starting on the left or right side and moving each file either to the left or right side until you only have one group open.

In case somebody wants to enable it (or the horizontal version):

UPDATE:
Googling around I was able to find numerous people complaining about all sorts of bugs with splitting/grouping code windows while using ReSharper. You could try removing ReSharper, ensure windows are not split/grouped, restart VS to verify if it worked, then re-install ReSharper?