Most likely cargo shifting in the bay. The most likely culprit for this is someone loading the cargo improperly, which was either improperly balanced in the first place or that broke free, slid around, and altered the balance of the plane.
It might be that the cargo was loaded properly, but that the weight was just too much. That would result in insufficient lift. The question might be; did the plane break resulting in an aborted takeoff that left the plane broken and off the runway?
Or did the takeoff get aborted due to insufficient lift, resulting in the plane leaving the runway and then breaking? A video of the takeoff would be quite enlightening.