To reproduce:

In a RadRichTextBox enter a line with the character 'a'. Press Enter,align the text to the right (using the ChangeTextAlignment method)  and type a short word. Press Enter and align the text to the left. Type another word. Export the document to RTF and import it. The last word you typed will be aligned to the right

Workaround:
Use a different exported, like HTML: http://www.telerik.com/help/winforms/richtextbox-features-import-export-import-export.html

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.