I've long believed that manual reviews play a bigger role in Google than is commonly thought. But I think that they are probably triggered by reaching a certain level of Google traffic rather than a certain position in the SERPs. For example, any site that starts getting 10,000 or more Google referrals per day will automayically be reviewed. (10,000 is just an example -- I don't know the real threshold)
I also think that several people are involved in the review process, so that the decision isn't just left to one person but must be affirmed by a supervisor.