pH effect in the hydrophobic interactions between two polypeptide chains
Here is one possible mechanism:

In tightly folded, compact, "globular" protein structures, the most hydrophobic bits of the protein tend to be in the core of the structure, away from the surface.  

If the pH changes, then by mechanisms you mention in your question, this structure can be disrupted.  If it is side chain interactions among surface-oriented, polar residues that "hold together" the globular structure, then a pH change can disrupt those interactions, allowing the previously isolated hydrophobic "core" to become  exposed.  This core can now bind more surfactant molecules, aggregate with itself, or do any number of other "hydrophobic" things.