There hasnt been a proper explanation of how the leak happened as Summit is refusing to comment about it. It looks like an early version of the script as it is in blue and over 110 pages long.
SPOILER ALERT. The first scene sees the introduction of Bryce Dallas Howard as the evil bloodsucker Victoria and her attempts to turn a young man into a vampire
It isnt teh first time it has happened as we all remember last years New Moon script incedent. Lest just hope it wont happen again!
Rest of the story can be found here