@uncollated - Even I couldn't figure it out at first. Then I found a clue in the second pane - The Focus Group was 'planning to storm the observation room' that PHB and Dilbert are in which is separated by a two-way mirror (ones used by the law enforcement agencies for questioning). In the third pane Dilbert can see that one of them has picked up a chair and is planning to thrash the two-way mirror. Hence, the panic on his face.
@uncollated - Even I couldn't figure it out at first. Then I found a clue in the second pane - The Focus Group was 'planning to storm the observation room' that PHB and Dilbert are in which is separated by a two-way mirror (ones used by the law enforcement agencies for questioning). In the third pane Dilbert can see that one of them has picked up a chair and is planning to thrash the two-way mirror. Hence, the panic on his face.