Click a button to expand/collapse all expanders.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows;
using System.Windows.Controls.Primitives;
using System.Windows.Media;

namespace Medtronic.NIM4.UI.Common.Converters
{
    public class ResetRadioButtonOnVisibilityBehavior
    {
        public static bool GetEnable(DependencyObject obj)
        {
            return (bool)obj.GetValue(EnableProperty);
        }

        public static void SetEnable(DependencyObject obj, bool value)
        {
            obj.SetValue(EnableProperty, value);
        }

        public static readonly DependencyProperty EnableProperty =
            DependencyProperty.RegisterAttached("Enable",
                typeof(bool), typeof(ResetRadioButtonOnVisibilityBehavior),
                new UIPropertyMetadata(OnEnablePropertyChanged));

        private static void OnEnablePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var element = (FrameworkElement) d;
            if ((bool)e.NewValue)
                element.IsVisibleChanged += ElementOnIsVisibleChanged;
            else
                element.IsVisibleChanged -= ElementOnIsVisibleChanged;
         }

        private static void ElementOnIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
        {
            var dependencyObject = (UIElement)sender;

            if (!(bool) dependencyPropertyChangedEventArgs.NewValue)
            {
                var checkBoxes = FindVisualChildren<ToggleButton>(dependencyObject).ToList();
                checkBoxes.ForEach(i => i.IsChecked = false);
            }
        }

        #region private static
        private static IEnumerable<T> FindVisualChildren<T>(DependencyObject root) where T : DependencyObject
        {
            if (root != null)
                for (int i = 0; i < VisualTreeHelper.GetChildrenCount(root); i++)
                {
                    DependencyObject child = VisualTreeHelper.GetChild(root, i);
                    if (child is T) yield return (T)child;
                    foreach (T childOfChild in FindVisualChildren<T>(child))
                        yield return childOfChild;
                }
        }
        #endregion
    }
}

<Border converters:ResetRadioButtonOnVisibilityBehavior.Enable="True"
        Style="{StaticResource SettingsSideBorder}">