When entering or exiting sidebar menus (explorer, bookmarks, etc.), the back button to return to the sidebar main menu is located on the right-hand side of the screen (next to the full screen button, which I cover here: Phone: Move Ribbon from menu to sidebar)
Furthermore, the icon used to portray the function to go back is two opposing arrows, instead of the usual single left-facing arrow. This conflicts with the usual interface design language found on most all smartphone UIs use, and is confusing as a result.
I would suggest either relocated and changing the back button icon, or entirely removing the main menu to choose explorer, bookmarks, etc. and simply add a top row of menu items as found on tablet/desktop (this would also make it consistent between all device form factors). Thanks.