When I first wrote this answer this trend was fairly new. Historically Hollywood movies opened in the USA first (weeks or even months before the rest of the world if you go back a decade or two). However by 2012 simultaneous releases had become the new procedure that the industry was experimenting with. Since 2012 it has clearly become the norm for movies to open somewhere in the 'rest of the world' before it opens in the USA.
The main enabling factor was technological changes in movie distribution. With digital rather than film distribution it is much easier and quicker to distribute movies. With the method of distribution being less of a barrier, you can now design your launch of the movie to meet your other needs.
One important reason is that you can build the reputation of a movie before it opens in the USA, one of the biggest markets in terms of revenue.
However another key factor in the minds of studio execs is preventing piracy cutting into profits. It used to be very hard to pirate a movie. You would often have to film a movie in a theater, and the quality would be normally pretty bad. A pirated movie on VHS or DVD would be notably lower quality than the product in the theatre.
Nowadays, with vastly better technology, it is far easier and cheaper for high quality pirated copies of movies to be made and quickly distributed worldwide. Studio's have to hit the worldwide market much faster to prevent pirated copies of the movie diluting their profits, especially around the world in markets where enforcement of pirated content is not so strong.