To reproduce: please refer to the attached sample video

1. Uninstall the already existing version of the Telerik UI for WinForms suite. Ensure that you don't have any installation and the Telerik menu is not visible in Visual Studio.
2. Open Visual Studio >> Tools >> Extensions and Update and search for Telerik WinForms VSExtensions. Install it.
3. After restarting Visual Studio, create a new Telerik project. You may be prompt to download the latest version during the creation process. 
4. Run the Toolbox configurator in order to obtain the proper version in the toolbox.
5. After the project is created, open the form's designer and drag a Telerik control from the toolbox and drop it onto the form. Open the Designer.cs and you will notice that control's declaration is inside the InitializeComponent method, not outside as expected.

The issue is not reproducible each time you follow the steps. It was reproduced with Visual Stidio 2012 on Windows 10. 

Workaround:

Download the Telerik UI for WinForms msi from your account and install the suite.