Firstly, if one pulls certain circuit breakers on a B747 the aeroplane thinks it's airborne. If the safety lock pins, which prevent landing gear retraction, are also not fitted to the landing gear then it is possible for the landing gear to try and stow itself when the gear lever is selected up (to physically select in the flight deck for the undercarriage to retract). The result of this, when done on when the aeroplane is on the ground, is that the nose gear will try and fold forward – trying to fold itself away. The main gear has too much weight on it (as well as the way it folds) to retract.
Secondly, if one unloads the freight in the wrong order, i.e.- removes the weight at the front first, then it is possible for the aeroplane to 'sit down' on its tail (more so with a B747 Cargo model). Similarly, if the two middle main (of the 4 main gear) landing gear are not 'high' enough, or pumped up enough, this can also assist in making the aeroplane 'tail heavy', and the nose will lift off the ground – putting the aeroplane nose in the air and sitting on its tail end. To assist with keeping the aeroplane level when loading and unloading freight there is a long jack that can be place under a jacking point at the tail. Now and again someone doesn't put this in place when loading and unloading. And now and again when you are doing something else with an aeroplane you just wish one of these jacks wasn't lying around where it shouldn't be.
Once every few years someone, somewhere, messes up...