Fling, I think the answer is the inertia of the fuselage - the lift force opposing the weight is suddenly removed and the acceleration of the fuselage towards the ground takes a short while to build. In that time the rotor blades are flapping down and coning down as the collective is bottomed so as Nick says the rotor head is trying to descend faster than the fuselage for a brief period of time.
Whatever the explanation, I know it happens - I have demonstrated rapid entry to auto in a helo with a g meter fitted and you can see the reduction - not to zero g by any means but a good bit less than 1 g which in a teetering head helo starts to reduce your control power.
I agree that you should always try to flare as you enter auto - the main reason being to try and recover the rapidly dwindling Nr (in the case of a real engine failure). And before anyone tells me I know that this technique needs to be modified for the low speed/climbout scenario.
Twiddle, no you don't need negative pitch for this, we are talking about normal everyday helicopters.