Desmond arrives at the infamous stadium where he met Jack in a flashback of the original timeline. He sees Penny, doing the same tour de stade that Jack and Desmond were doing at the end of season 1.
Desmond slow approaches her and has a big smile on his face when he introduces himself. He shakes her hand, and at that moment he flashes back to the original timeline, in the electromagnetic box, alive and well.
It seems that this pivotal meeting between Desmond and his constant, Penny, pushed him back into the original timeline. Could it be because he achieved what he was destined to achieve in the other timeline to accomplish what he needs to in the original timeline?