Trailer editor here. This usually happens when the tracks are split out for 5.1, 7.1, or in the case of IMAX with lots of speakers and subwoofers. Each track of audio (dialogue, SFX, music) has to be routed to different speakers. The dialogue is almost always output to the center channel, and in the case of IMAX, there are many directions. This happens when the final mix is not mixed down properly, and in this case, it wasn't mixed down correctly for web usage. I have never seen something this huge happen, as these things usually run through many levels of QC before posting.
