The Fallen didn't universally turn to Chaos, though. It was just a few key figures at first. Later, others fell to Chaos. Hell, there might even be full on loyalists among the Fallen. There was plenty of strife among the DA that were on the planet when the Fallen rebelled.
The DA Fallen are the most ambiguous out of the Astartes in terms of what they are. The general idea of the Fallen to begin with was that they needed to stand for Caliban and its people, that the Imperium was a poison that needed to be purged. And, whatever his motives, Luther had a lot more respect from the Knights of Caliban, and when he rebelled, they followed.
The answer was ambiguous because the answer is ambiguous.