What I would suggest you is to create a ViewModel where are all the transitions among which you would like to select a random one, a property for the selected transition (which the Transition property of the RadTransition control will be bound to) and when the Content is changed you can change the transition. You could refer to the following blog post, which implements the idea - http://blogs.telerik.com/SilverlightTeam/Posts/10-10-14/perspectiverotation_and_mappedlight_transitions_preview_of_telerik_q3_2010_-_radcontrols_for_silverlight.aspx.
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