Another Possibility
Maybe this is more of a question for a 744 Eng/Airframe LAME.
Maybe the wing to body fairing may have been loose enabling the airflow to rip it from the airframe. I noticed that the portion that is still intact is possibly tied to the airframe using tie-rods. Does the missing part also use tie-rods? If this is so, as the fairing seperated, maybe the tie-rod mounts damaged the pressurised area as they were pulled away (thus causing the explosive decompression). Seems strange that the fairing has come away clean (screws and all) on the fwd-most area.