tartare, the usual cause of rotor separation due to mast bumping is a result of low "G'. In this case the rotor hits the mast at the nine o'çlock position and separates. Because the blade on the right is travelling forward and that on the left is travelling rearwards there is a dissymmetry of lift between the two blades, the right having greater and the left less, the disc then rolls left with respect to the fuselage and impacts the tail boom 90° following separation ie .046 of a second after separation, rotor RPM being 324.
A video on the subject.
Had the unpleasant experience of being on the side lines of a fatal mast bump in Vietnam. We had a formation of five aircraft for a troop lift, but on arrival only four were needed, me being #5 was delegated to suntan duty. I was on my second day of flying duty having just returned from ten days R&R in Hong Kong with my wife and anxious to get back into it, so tried to swap with #4 and let him suntan. Unable to persuade him he went off and some short time later he had a mast bump. One take off technique used was to accelerate to Vne on the tree tops and then cyclic (zoom to FW folk) climb to altitude. The push over at the top of climb was where you had to be careful not to get into a low "G" situation. Often used when you were heavy and/or likely to encounter gun fire. The wreckage was winched into the back of a Chinook and delivered home
The accident in question.
Information on helicopter 68-15568