My Carhartt jacket only softened up once it started becoming sun-bleached...

I suppose any sort of treatment that would aid to break down the fibers would aid in the softening...

I don't think starch itself can do that - unless there is something in the solvent for those starch products... Maybe a little bacterial/fungal action - could be those lil' critters use the starch as food and release something into the fabric...

I just found this....

Q. What are the ingredients in Niagara Spray Starch?
A. Niagara spray starch is made of modified corn starch, for body, silicone to help prevent the iron from sticking, fabric softeners, borax to keep the starch from breaking down, preservatives to prevent microbiological growth, and fragrance to give a pleasant smell.

And this:

For example, the STA-FLO brand lists not only water and corn starch as ingredients, but also (in order of concentration?) borax, "processing aids," preservative, "ironing aid" and perfume.

So, perhaps it is the fabric softeners or the borax... I would hypothesize that the starch keeps the fabric softeners in contact with the fabric longer... Or that borax may do something to the fabric......

Homemade spray starch contains:
Spray Starch

1-3 Tbsp. cornstarch

2 c. cold water

Using funnel, mix ingredients well in a spray bottle. For a light starch, use 1 Tbsp. cornstarch, for a heavier starch, use 3 Tbsp. Shake well to dissolve cornstarch completely, keep out of reach of children. Makes 2 c.

So anyway, I really don't think it is the starch itself, but maybe the softeners or other chemicals, or even the action of 'wrinkling' and 'working' the fabric.....