The climb got progressively steeper before the nose suddenly dropped. The dropping of the nose is consistent with the pilot pushing forward on the cyclic control in an attempt to recover from the nose-high attitude. The pilot should have been aware of the dangers of mast bumping through the pilot's initial helicopter licensing training and subsequent experience with helicopter operations. It is likely that, in the pilot's preoccupation with the open door, he did not apply the corrective actions required to recover from a nose-high attitude. This allowed the helicopter to enter a low-g condition, which in turn led to mast bumping and the in-flight breakup of the helicopter.
This paragraph seems to be contradicting itself. Having ASSUMED, the climb became progressively steeper (though the first sentence reads as if it came from FDR data) the board suggests that the pilot's sudden recovery caused the reduced loading -
the pilot pushing forward on the cyclic control in an attempt to recover from the nose-high attitude
But later they imply the low-g condition developed without pilot input -
It is likely that, in the pilot's preoccupation with the open door, he did not apply the corrective actions required to recover from a nose-high attitude
I'd go with the first statement - the pilot propably found himself with no airspeed point skywards and 'shoved' the cyclic forward more quickly than was wise.