@IPC, @getnkk
even if you have not changed employers, if it has been a while since you applied for I485, you have good chances of getting a RFE for Employment letter (the idea being that the offer for that position is still valid after all these years).
so watch your e\mails closely.
usually the triggering points are when they have reason to touch your files e.g.when you apply for EAD or AP and now it also seems Change of address and AC21s are added to that list. On top of this, NSC's pre-adjudication agenda has added momentum to this process.