I've been paying attention when this happens to me and I don't know for sure, but I think what's happening based on experience and patch notes is:
Enemies start with an inherent inaccuracy and unpreparedness to prevent the "instant crackhead headshot" that were so common before. Once an enemy "engages" you, their accuracy and readiness increases at a predetermined rate per level. If you "disengage", their accuracy and readiness don't decrease so the next time you see them they're totally capable of the "instant crackhead headshot". I'm guessing this guy either saw you earlier or being so close to a gunfight set him to "engaged" mode and he no-scoped you.