AC-21 Rule is Not Available for Alien Beneficiaries with Evidence of Actively Searching for New Employment
When the alien beneficiary voluntarily departs from the employment in an hostile environment to get a new job, and the USCIS obtains an evidence, either before 180 days or after 180 days of filing of I-140 and I-485, the foreign worker may face a risk of denial or revocation of the petition, because of the evidence of such alien's intent not to work for the employer for the petitioned job, and the AC-21 rule is not available for alien beneficiaries with evidence of actively searching for new employment. In this case, foreign workers who had departed from the employment, not because of the layoff, or because of the alien's decision to change employment.
Such adverse evidence can also haunt after the foreign workers obtaining the Green Card as the law allows the USCIS to initiate the Green Card revocation proceeding before the immigration courts under the law that the USCIS can revoke a Green Card, should they belatedly find and establish such adverse evidence after the approval of a Green Card, which should have formed a basis for the adjudicator to deny the I-485 applications had the adjudicator known the facts and evidence.
The issues here involve in most cases hostile employers or other third parties who possess such evidence, and offer to the USCIS to hurt such foreign workers. Usually such denial or revocation is preceded by the USCIS' initiation of a notice of intent to deny or revoke (NOID or NOIR) when such action is taken before the Green Card is approved, but when a revocation proceeding is initiated after the green card approval, they file revocation proceeding before an immigrant court as such alien is entitled to a hearing and decision by an immigrant judge.
http://www.greencardapply.com/news/news09/news09_0210.htm