8 oz in 2 gal of water, applied over 50 sq ft is equivalent to treating 1 surface-acre with 54.45 gal of 2,4-D in 1,742 gal of water-carrier. That is a lot of chemical and a lot of water-carrier!
The first question is: What strength was the 2,4-D Amine concentrate?
Assuming the 2,4-D was labeled for aquatic-use (?), it would have contained 4 lbs of active-ingredient per gal of concentrate. If that is correct, your application was grossly over the labeled rate, and the plants are probably reacting more so to "chemical shock" than to the product's typical mode of action (unchecked cellular elongation - aka "the plant grows itself to death").
IMO, a topical application of HABITAT @ 3/4 oz per gal of mix-water, would have been a better option - since I doubt if a liquid 2,4-D topical-treatment will provide thorough root-kill of water-shield (granulars attack through the root-system).
Also, I don't think vegetable oil is water-soluble. A methylated seed oil, in a water-soluble formulation, is a much better choice for wetting and leaf-cuticle absorbtion (@ 3/4 oz per gal of mix-water).