Most of the explanations for Johnson's false starts are from Eagles fans, writers, or apologists.
Here is the rule: "It is a False Start if the ball has been placed ready for play, and, prior to the snap, an offensive player who has assumed a set position charges or moves in such a way as to simulate the start of a play, or if an offensive player who is in motion makes a sudden movement toward the line of scrimmage. Any quick abrupt movement by a single offensive player, or by several offensive players in unison, which simulates the start of the snap, is a false start."
Lane Johnson's leg kick, as they call it, is the beginning of his backpeddle. It is absolutely the simulation of the start of the play. If he does this before the center moves the ball to snap it, then it is absolutely a false start and should be flagged. There is no exception to this rule as Eagles fans claim. As we have seen many times, even the slighted move before the snap by an offensive linemen once the line is set results in a false start. It is ridiculous to argue that a center or even running back who wobbles less than an inch commits a penalty but Lane Johnson moving his back leg up into the air before the snap is not.
I would love to know why the league does not throw a flag on him when he does this.
At the same time, I wonder if his leg kick tips off the defense that a pass is coming. Does he do this every play or just on pass plays. Seems to me if a run is called his first step would be different.