can a H1b petition be revoked after status says post decision activity

I saw a case on the USCIS website whose status is Post Decision activity and it says On November 19, 2013, the previous decision made on your case was revoked and we mailed you a notice explaining the decision

Could this happen to cases that were approved? Can it still be revoked once they send an approval notice?

YES, AFAIK only approved petitions can be revoked.

What could be the possible reasons? Wanna know for my peace of mind

May be ur employer revoked. Contact him