Fluctuating descent path

When the descent path fluctuates like this what could cause that?

I've seen this a couple of times but I never really understood what causes it. In this case I checked both my legs and winds and all seemed normal.

I noticed this already prior to TOD where the T/D indicator on the NAV also jumped around and then after starting to descend it stopped after a while all by itself without any intervention from me.

https://dl.dropboxusercontent.com/u/94801887/20130525_174038.mp4