"-던" may imply a temporal interruption in whatever action or process in question. In all the aforementioned examples, their respective "-던" clause either precedes and introduces or describes a rupture in time, an interruption of some sort or just an event that happens unexpectedly, counterintuitively, etc.
"그러던" by itself in your given example implies first, how things were going normally, predictable and second (thanks to "-던",) introduces a rather sudden, unexpected interruption or just new information that has not been mentioned. Could you clarify what is confusing you in particular?