Throwing the legs to the side is something that the horse does if for whatever reason they can't tuck their hind legs up tightly enough to clear the jump otherwise. It indicates one of two things: 1.) That the horse lacks the ability or scope cleanly. You'll see Grand Prix jumpers do this over a maximum height or maximum spread or 2.) something in the horse's conformation or soundness prevents them from folding their hind legs in the normal fashion.
Since you say this happened over 2' 3"; I'm going with #2. Posting photos or a video of her jumping would be a big help.
Exactly, and going to add that rider imbalance/leaning can cause this, as well as if your jumping at an angle and/or asking the horse to turn in the air for a landing and sharp turn, or the horse is anticipating a sloping/uneven/poor footing landing.