It can be useful to think of force couples here.
For a teetering head where the fuselage pretty much just hangs off the mast, you might wonder why disc tilt would change body angle at all, but I think of it like this:
Start in steady hover. CG hangs under mast head.
Disc tilts and starts to move forwards, dragging mast head with it. Fuselage drag increases, giving a rearward force centred lower than the mast, pointing back.
The couple between the forward force at the head and the lower rearward drag causes a forward rotation around the CG.
This rotation will continue until an equal and opposite couple stops it.
With the fuselage tilt, the CG moves rearwards from out under the mast. Now weight pointing down from the CG and lift pointing up at the mast head have made an opposing force couple. The fuselage will keep on tilting forwards until the the two couples are equal and opposite.