I'm not sure why your RadRibbonBarGroup is extending in the row as opposed to down the column. This may have something to do with how you are declaring your RibbonBarItems inside the RibbonBarGroup. Take a look at the examples here
and see if it helps you format the way your RibbonBarGroup looks.
If this does not solve your issue, it may be important for you to send the code in question so that we can see exactly what you are doing with your RibbonGroup.