Probably because you have too much in your formula already and adding more brings the salt curve down. Some surfactants have salts in them already but it’s impossible to say anything without seeing what’s in your formula.
Hydroxyethylcellulose will not support monovalent electroltyes (e.g. SLS, ALS) or amphiphiles (amphoterics) in high amounts well at all. That is why your viscosity was unstable even before you added NaCl. Try methylcellulose (Methocel from Dow) instead, you’ll feel much better.