The UpdateSourceTrigger property of the column's DataMemberBinding is ignored.
The reason for this behaviour is that the RadGridView overrides and changes the "Binding.UpdateSourceTrigger" property to be "Explicit". It is designed like so because the editing and validation mechanism of RadGridView explicitly updates and validates values when the edit is over. Otherwise validation and commit would happen on every key stroke.
You could overcome that with a few lines of code using GridViewColumn.CellEditTemplate property and setting binding for the element in Edit mode (for example the editing element for GridViewDataColumn is TextBox).