I'm mainly referring to movie settings/themes. It seems that the Western setting/theme is becoming the hot item right now, with True Grit, Cowboys and Aliens, Rango, and The Warrior's Way (kind of). These sort of odd spurts in a particular movie theme or setting seem to occur every couple of years with regularity. Sometimes, they are very obvious (Dante's Peak and Volcano......Deep Impact and Armageddon..etc.), and sometimes rather subtle (within the superhero movie genre, movies focusing on the regular person being a vigilante in costume became popular early this year with Kick-Ass and Defendor)

Why do you think this sort of thing happens, what research prompts it, what are the best things that have arisen from it, who are the major trends setters, and what are some great examples of this? Answer as many or as few of the above questions as you like. I'm just curious to see your take on this pattern of film making.