This wasn't an accident. It was a marketing stunt to get some buzz around an otherwise unnote worthy movie release.
See. It's working.
Maybe the file was mixed in a directory with trailers and the person uploading simply chose the wrong one and moved on without verifying.
(Or replace "never" by "always" if you're feeling kind and positive)
Edit: To be clear, I don't believe in either philosophy as an absolute, it just seems appropriate here :)
If you can't adequately explain intentions with available facts then you should say "I don't know" and leave the case open. We can speculate on why it was leaked but we really don't have enough case-specific facts that can shade light on the events that lead to this "leak".
Of course this specific case is not important to most people,but my point was regarding the accuracy of probability as more unsolved variables as well as yet unknown variables increase. If we knew specific individuals or had some track record behind the specific organization, I say logic like "Hanlon's razor" can be evaluated usefully.
If this was a more controversial case (think rodney king murder trial),it would have been more interesting. Since after all, an action is not a crime without criminal intent. Whether malice,negligence or incompetence assumptions can have an adverse impact on all involved parties.
Within the context of movie marketing is stupidity more usually the reason for a media megacorps release of media or is marketing more usually the reason? Surely then the probability lies firmly with marketing until evidence shifts that assumption.
Razors are not truth generating though they are often treated as if they were.
It has been available to torrent for some time. I can't think of a better candidate to try this experiment on. It worked fantastically. Beyond that I can't see this having gone unnoticed for 6 hours.