I think it is safe to say that the fish mortality is unrelated to your 2,4-D lawn-treatment. Beyond that, I really can't speculate as to what else might be the culprit.
Indeed, 2,4-D labels make the statement "...may be toxic to fish and aquatic invertebrates". However, the intent of that statement applies to extremely high concentrations (ie spills, excessive/over-appliations, etc). Label-sanctioned aquatic treatments, and labeled turf-applications, are not indicted as a direct threat to fish (though they could possibly represent indirect-threats due to DO crashed).
