To reproduce:
Add a RadRichTextBox to a form, insert a table. Enter a cell, press the down arrow - cursor is moving right, press the up arrow - cursor is moving up. If it reaches the end of the row then moves down and if the start is reached - moves up.

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