Its not a bleed valve closing at a set point?
I don't think so, the bleed valve in the gas generator turbine closes fully at 80%Ng. The peak is roughly at 20% of the engine's full power.
I'm leaning for this one:
As you advance your power lever, particularly at lower Ng's(N1's), your increase of fuel to the fire is momentarily ahead of the increase of airflow.