Increasing speed reduces the angle of attack on the prop blades.

If you had a fixed pitch prop this would mean that you lost thrust, and the faster you went the less thrust you would have.

But you have a constant speed prop, and as the angle of attack reduces the torque drag reduces and the prop RPM begins to go up.

The CSU senses this, and increases the blade angle to get the RPM stabilised again.

So you settle down at the higher speed with increased blade angle.

Think Hurricane Mk I, fixed pitch wooden prop with a horrendously coarse pitch for high speed.