What's going on here? A number of factors surely play a role in this phenomenon. However, I think the general point is that teams have a tendency to be inward-focused at times. An in-group vs. out-group dynamic emerges, whereby you exhibit an affinity for your fellow group members, and you tend to reject, marginalize, or discriminate against those in the out-group (such as the outsider providing input to the pair). The group members also may spend time bolstering each others' confidence in the judgment at which they arrive, and that makes it difficult to alter that judgment in the future.
I recall one fascinating example of this phenomenon in action during a leadership development workshop. My colleague Amy Edmondson was conducting a team exercise called the Electric Maze. She invited a group of individuals on stage to work on the exercise. After the group had a chance to plot their strategy for a few minutes, the audience members had an opportunity to offer the group advice before it started the exercise. The group barely listened to the audience. They had become so fixated on the strategy that they had begun to concoct that they were not receptive to outside advice. The amazing thing is that the group had only been plotting its strategy for a few minutes when the outsiders chimed in with their input. Yet, the group dismissed the outside input. The team already had become insular!