One reason for this polarisation is that, as a group, people can come up with a bigger set of persuasive arguments in support of the biased options: because everyone favours leaving the job, everyone suggests reasons to do so. But they come up with slightly different reasons. One person may point out that your friend is unlikely to get promoted any more at the bank, another that a new job would mean he’d meet new people, another that he never has the chance to travel in his current job and so on. So by the end of the discussion, all the group’s talked about is a lot of good reasons in favour of one option. As a result, the group agrees on a more extreme conclusion based on this surfeit of good reasons.