In data binding scenarios these propeties are not updated automatically. Currently if you want to bind the StartPoint and EndPoint properties of the RadDiagramConnection you will have to update new position of the connection manually. This can be done in the PropertyChanged event of the connection. In the Handler you can cast your connection to IConnection and call the Update() method, if the e.PropertyName holds the property that you change.

Will be available in Q1 2015 Release.