The stacking of toolbars sometimes occurs after the following sequence of steps.
1. Dock a toolbar beside the Standard toolbar so that they are both horizontally docked below the main menu toolbar.
2. Minimize and restore ArcMap.
The toolbar may jump to the next row below the Standard toolbar; however, this doesn’t happen consistently.
This is a known issue caused by a bug within the Microsoft MFC runtime DLLs. The same scenario occurs within Microsoft WordPad, which is also MFC-based. ESRI is currently working on a resolution.
The only known workaround is to drag the toolbar and dock it in the previous position.