In Act II, Macbeth is beginning to think and plot on his own. Up until now, Lady Macbeth has made all the plans (the murdering of Duncan, the way to received the guests knocking at the door, "a little water clears us of the deed," and so on). She has been the coach--both drawing up the plays and directing the players for the greatest success. Macbeth followed her guidance.
Now, Macbeth does not consult his wife. He simply hires people to kill Banquo and Fleance. He sees Banquo's ghost at the banquet table. His wife still thinks it's the guilt of Duncan's murder weighing heavy on her husband's heart, but she is not in the know. Macbeth is plotting behind her back, and he is withdrawing from her. They are not the close-knit couple we saw in the beginning of the play, and she is losing her control--both of her husband, and later, as we will see, of her own faculties.
In essence, they change roles. She was more the man wearing the pants in the beginning of the play. She called the shots, and he followed her lead. Act II is the beginning of the gender role reversal (or back to its "natural" place) where she takes the more submissive female role, completely unaware of Macbeth's dealings, and he steps into the pants where he makes the plans and calls the shots without consulting her or informing her of his evil plotting.