The Agusta 109 rotor brake control is a badly thought out design, certainly in the way that it's not pilot proof, as this one discovered.
As the report states, the handle on the left side of the overhead panel is fairly small and is partly hidden by the ECLs. The latter are normally left in the "Flight" position at all times (engines are started and stopped using the controls on the centre console) so there is no need for putting one's hand up to move them - which might be a further prompt to notice where the brake handle is.
The utility hydraulic pressure for the rotor brake tends to bleed away after shutdown, as does that of the parking brakes (it's important to chock a 109 if you want it to be where you parked it on a slope when you return).
If there's no utility pressure the rotors will begin to turn quite normally on start up despite the brake handle still being in the "ON" position. However, as the rotors begin to turn, the hydraulic pump re-pressurises the utility system and the brake comes on again all by itself, harder and harder... say no more!
I never leave the rotor brake handle "ON" for this reason. I know of one pilot some years ago who started his first A109 job in the morning, inadvertently left the rotor brake on for a subsequent startup and was looking for another job by the end of the afternoon.