文本框上的WPF边界刷一旦在IsMouseOver触发器上设置就会消失

I have the below XAML which attempts to set the border of all TextBoxes in the window to red OnMouseOver. What happens is when the mouse is over the textbox the FontSize and Foreground properties are set but the BorderBrush is only set momentarily before reverting to its previous default value. I want the BorderBrush to stay red until the the mouse is no longer over the textbox. Any ideas why this happens?I have the below XAML which attempts to set the