Whilst it may seem an obvious cause, the report doesn't say why/how the door opened nor does it actually report that the pilot was trying to close it.
Noted, but comments on it are intended to bring it to the attention of other pilots in order so that they don't make the same mistake.
Jimball - By speed I meant forward motion to help keep the door closed and for the heli to become a bit more stable.... not 100 kts. Although, I have never had the experience of the 44 door being sucked open while at speed while demoing this with students... & by height, I meant get away from the ground.