Humic acid isn't one molecule, so what does it mean to make its "sodium salt"?
Humic material contains many acidic groups. When treated with base, groups such as -COOH lose H+ and become negatively charged -COO- sites associated with Na+.
Those charges make a big difference.
They increase interaction with water and electrostatic repulsion within the humic material, making sodium humate much easier to disperse in alkaline aqueous systems.
Acidify it again and many sites regain H+. Charge decreases, aggregation becomes easier, and humic acid can precipitate.
It's ordinary acid-base chemistry acting on an extraordinarily complicated natural mixture.
Global Chemical Database