The way I had it explained to me was it happens because of Flap back.
It is easier to see with a picture but if you can imagine a two bladed machine in forward flight. This machine is producing lift, both blades are producing the same lift but have different AoA's and airspeeds.
Simple so far right!
Now put some values on lift for simplicity I will use small numbers
We currently have a lift value of 20 on each side.
Advancing side has a higher airspeed and lower angle of attack
Airspeed = 10 AoA = 2
10 x 2 = 20
Retreating side has lower speed and higher AoA
Airspeed = 5 AoA = 4
5 x 4 = 20
Now we lower collective by 1 degree
Now advancing side is
10 x 1 = 10
Retreating side
5 x 3 = 15
You have more/less lift on the retreating/advancing side this precesses 90deg to make nose pitch down.
So while flying we automatically correct for flap back with cyclic, but this setting only works at a given speed/collective setting. if you change either of them you have to reset the cyclic.
I am sure there are probably many factors to this question but this is my main understanding of how this works.
Crispy