Creating a large page with RadEditor control and opening the mentioned dialog will scroll the page to the top. This causes the user to loose visual  contact with the Editor.

The issue is due to the automatic textarea focusing in the clientInit() method of the dialog's logic.

To resolve the issue you should modify the dialog and remove the focusing functionality. To do so follow the next steps:

    1. Follow the Custom Built-in dialogs approach (http://demos.telerik.com/aspnet-ajax/editor/examples/externaldialogspath/defaultcs.aspx) to use an external and later modified Plain Paste Dialog - the MozillaPasteTextDialog.ascx;
    2. Find the clientInit() method trough the client-side logic of the MozillaPasteTextDialog.ascx file;
    3. Remove the this._container.focus() line.

You can also find attached a sample page with the fix placed, so that you can examine it.