I'm not sure if it is purposeful but certain types of movies all seem to get released around the same time. Most recently I remember a group of crime thrillers all released with in a month of each other: Pride and Glory, Body of Lies, and Righteous Kill.
Recently the box office has seen great success with children's movies: Chronicles of Narnia, Bridge to Terabithia, Nim's Island, and Race to Witch Mountain. Not to mention the monster hit that is Harry Potter.
Trend or not, I am SO excited for children's book turned film, Where the Wild Things Are: