I've seen this happen before too. I don't remember if I placed anything in the scene between the first time I tried to place the mayor in the chair and the second. I do remember that in one case the mayor would not sit down and later would. The game's idea of when a scene is too crowded for another element may be a bit fluid, or might be related to your zoom level or something changeable like how many bugs or NPCs are in the scene. I am just guessing here.