The built-in commands perform their function without any programming on your part. If you are not adding any custom commands to the RadDock control, you can specify which built-in commands using the DefaultCommands property.
DefaultCommands can be set to any of the following values: None, PinUnpin, ExpandCollapse, Close, All. You can build your own combinations, e.g. DefaultCommands="Close, ExpandCollapse". In your particular scenario you can specify only the expanding behavior by setting DefaultCommands = "ExpandCollapse".
You can also set the DefaultCommands property which controls the behavior from server - as shown below: