Lady Macbeth may actually faint in astonishment that her husband has killed the guards in an act that was not part of their plan in murdering Duncan. She could be shocked that he has acted so boldly when she had to question his manhood in order to provoke him to kill the king.
On the other hand, she may be faking. Recall that Macduff was relucant to tell her about Duncan's murder because the news was too horrible for a woman's ears. Now she can convincingly pretend to faint at the news of the guards' murder when in fact what she is doing is drawing attention away from Macbeth, who has acted rashly and done something that wasn't part of the plan. Notice that Macduff immediately questions why Macbeth killed the guards. Why would Macbeth kill the guards? Right away Macduff is suspicious, and Lady Macbeth sees his concern so perhaps she pretends to faint, to be a delicate woman, so that people will turn to take care of her. Given her behavior earlier in Act 2, it is unlikely that she really faints because she is a strong woman.
As Shakespeare's Macbeth is a play, not a novel, no narrator gives us a reason for why Lady Macbeth faints, and thus we can only speculate on the cause rather than offer an authoritative answer.
As Lady Macbeth has spoken earlier of the need to "unwoman" herself to support her husband's ambition, we can argue that what we see here is a tension between her innate womanly nature as moral and gentle and the deeds she must do or support in her role as a loyal wife. This is the tension that eventually leads to her insanity and suicide, and we can see the fainting as foreshadowing it, an expression of unbearable inward tension.
Another possibility is genuine horror and remorse concerning the death of Duncan, especially as she hears the description of his body:
Here lay Duncan,
His silver skin laced with his golden blood;
And his gash'd stabs look'd like a breach in nature
We might remember that Lady Macbeth could not bring herself to kill Duncan because Duncan resembled her father. This horror returns as she hears the description of his body.
It is also possible the fainting is a ruse to deflect possible suspicions about the murder.