I guess no-one here watches on ESPn? Not that I love it, it's just what I get with my cable provider, but there was a great discussion between Jason Goodall and Brag Gilbert about her serve, in fact they both were standing up with racquets and illustrating service motion.
I love this kind of thing--I'm very obsessed with stroke mechanics etc.
Anyway, Goodall showed in video replay how there is way too much going on once she is in "trophy position" (racquet raised over her head, knees bent, way too many shifts in the angle of the racquet face, just too much going on. When it all works, it's an amazing serve, but it's flawed because there are too many things that can go wrong. When she's nervous.
Then once you the DFs start piling up you no longer trust yourself, and the lack of confidence makes it even worse.
Strange no-one has corrected this. But it is a big deal to tamper with your service motion.