We managed to reproduce this issue in the UserDockForm. It will be addressed in one of our future releases.
Still, for the time being, there is a workaround you can use to achieve the desired functionality. Store a reference to both forms (the one just becoming visible, and the one being hidden), and place a condition in the event handler so that the VisibleChanged event is handled only for these two forms.
Your Telerik points have been updated for reporting these issues.
the Telerik team