They didn't want it to get hurt by the Hunger Games. They are basically throwing Frankenstein into the trash.
http://deadline.com/2015/06/fox-switches-the-martian-and-victor-frankenstein-release-dates-1201441061/
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The move to October keeps the film away from the second weekend of the popular Lionsgate franchise Hunger Games: Mockingjay, Part 2 and off of the same weekend as Disney’s Good Dinosaur"
As for Frankenstein "This is the second time the film has been pushed. It was originally skedded for January. Enough said."