IIRC there are a couple of reasons:
1 If the disc itself tilts forward then it will start moving in that direction as the thrust vector tilts, hence it will induce a nose-down rotation in the fuselage about the aircraft's CofG because the directional thrust is applied offset from the CofG point (in effect dragging the a/c by it's hub).
2 In an articulated head (similarly to a rigid head, as you have said, but unlike a semi-articulated/teetering head) there is the additional effect of a rotating moment applied to the rotor hub itself as a result of the flapping hinges being offset from the centre of the (rigid) hub.
I stand ready to be corrected though!
BM