Rule 7, Section 4, Article 8
It is also an illegal shift if a player under or behind center goes in motion and fails to come to a complete stop for at least one full second prior to the snap, or before a second player goes in motion.
Also note that this would normally be an illegal shift, but if it was under a minute and the clock was running, if the illegal shift is for all 11 players failing to be set, the penalty would become a false start penalty with a 10 second run off.