Personally, I don't think it was a trident as you can't hit a plane with it. Seems more likely it was an SM-6 which the Navy has been secretly working on adapting onto it's subs. Pic attached is of an SM-6 launch.
Still makes no sense. THAAD is an Army program there are none stationed at Whidbey. While several submarines with vertical launch tubes were present in the area around the time of the incident so I'm sticking with that.
SM-6 would probably be easier to target by an F-16 than a D5 since it is a Radar emitter.