In a combobox the selectionChangedEvent is called when a value is selected from the combobox and also when setValue method is used. There is always a requirement that the event should be called only when a value is selected from the list than when programmatically setting the value.
One idea i was given when going through Sencha forums is to disable all the events before setting the value and then enabling them back again (this is a little weird, as this requirement comes up most of the time. They should probably try to make it a little easier than that)..
But anyways it works for now.
1: combobox.disableEvents(true);
2: combobox.setValue(); //set a value
3: combobox.enableEvents(true);