Re-reading the report, a container of oil was broached allowing oil to escape (onto the floor of the aircraft).
Other items of cargo then slid towards the tail, moving the CofG rearwards and lifting the nosewheel until the tail contacted the tarmac (or concrete).
During the unloading process, the oil spilled from its container in the center of the plane, coated the floor, and caused all the other cargo to “slump” toward the back of the cabin.
Due to the shifting weight, the plane’s nose lifted a few meters in the air, and its tail touched the apron.
Obviously the objects that slid on the oil were 'unsecured' (not strapped-down or not on secured pallets/cargo containers).
Whether the aircraft had flown with the load unsecured is a matter of conjecture and the consequence of movement in the air is frightening.