Hi,
You are redefining the controlTemplate of the DropDownButton. So you are modifying the behavior of the DropDownButton to act as you defined it. To keep the Behavior of the DropDownButton from the toolkit, you need to keep the controlTemplate of the DropDownButton and start your modification from this one. Here is where you can find the ControlTemplate of the Toolkit's DropButton :
In windows7 : Xceed.Wpf.Toolkit/DropDownButton/Themes/Generic.xaml
In windows8 : Xceed.Wpf.Toolkit/DropDownButton/Themes/Aero2.NormalColor.xaml
You are redefining the controlTemplate of the DropDownButton. So you are modifying the behavior of the DropDownButton to act as you defined it. To keep the Behavior of the DropDownButton from the toolkit, you need to keep the controlTemplate of the DropDownButton and start your modification from this one. Here is where you can find the ControlTemplate of the Toolkit's DropButton :
In windows7 : Xceed.Wpf.Toolkit/DropDownButton/Themes/Generic.xaml
In windows8 : Xceed.Wpf.Toolkit/DropDownButton/Themes/Aero2.NormalColor.xaml