Dis Mystery has it right.
The instantaneous reaction to lowering collective is for the lift on the advancing side to reduce MORE than the lift on the advancing blade. [The amount of blade angle reduction on adv and ret sides is reduced by the same but the ratio of lift is different]. Move this round 90 (ish) degrees due to flapping to equality and the front of the blade dips - helo noses down.
At the same time, on SOME helos, this goes one step further with a change in C of G dynamics.
Once the helo starts to descend.....then the tail stabiliser comes into effect -exacerbating this dip with the nose.