Thank you for your interest in the RadControls for Silverlight.
Actually the expander's animation is disabled by default. To enable it you should set the AnimationManager's IsAnimationEnabled
property to true for the chosen expander. More information and code snippets you can find in the Expander Animation
Attached you can find an example project which demonstrates the animation behavior of the RadExpander control.
Please give it a try and let us know if you have more questions.
the Telerik team