Button Animation

To answer the WHY part of your question, the value of the property set by animation takes precedent over setting the property directly, as described here.  To do what you originally tried, you have to clear the animation, as described here.

Here's as example (though I didn't use a StoryBoard)

        void Animate_Click(object sender, RoutedEventArgs e)
        {

            DoubleAnimation myDoubleAnimation = new DoubleAnimation();
            if (Animate.Width < 300)
            {
                // Animate the Button's Width.
                myDoubleAnimation.From = 75;
                myDoubleAnimation.To = 300;
                myDoubleAnimation.Duration = new Duration(TimeSpan.FromSeconds(5));
                Animate.BeginAnimation(Button.WidthProperty, myDoubleAnimation);
            }
            else
            {
                Animate.BeginAnimation(Button.WidthProperty, null);
                Animate.Width = 75;
            }
        }