Frankly, I would expect fly ash (being mostly various oxides) to act as a granular filler - increasing surface hardness and possiblity stiffness of the polymer - rather than as a plasticizer, as it should not be able to interact in a meaningful way (e.g. produce strong hydrogen bonds) with PP.

If anything, perhaps the addition of fly ash somewhat mitigates the stiffening caused by the sizal fibres, by affecting their distribution in the polymer, but this would be just speculation.

Can you perhaps provide further information about your system and achieved results?

It is possible that fly ash enhances the flow properties of your mixture owing to its spherical shape, something similar to using steel spheres that reduce friction in mechanical systems through the ball-bearing effect.

It can be used as plasticizer and improve elongation at break and enhance flowability with reduce slightly hardness for PP matrix