Siegenthaler is a good player, all defense, decent physical game, good fundamentals. He never stood much of a chance this year. The Caps had already loaded up on D. Then at the 11th hour, Chara fell into their lap at a bargain price. Chara took Siegenthaler's spot.
The Caps have remained stubbornly healthy on defense all year. Couple that with the fact that Laviolette wants the defense to play an up tempo style in which the defense pinches and activates the transition game, and there was never any room for Siegenthaler on the Caps. As the team's style of play has shifted, Siegenthaler is no longer the ideal fit that he would have been under Trotz.
Prospect defensemen Martin Fehervary and Alexander Alexeev are knocking on the door, and Michal Kempny will be back from injury next year. It was clear there wouldn't be a chair for Siegenthaler soon, even as a seventh D. Best of luck to him in NJ, where he should get the playing time to develop into a fine NHL player.