I'm not sold on herbicide damage although it's possible. Rapid growth from 2,4-D or dicamba (both part of standard lawn spray) overspray causes epinastic growth which results in twisting and cupping. These herbicides are IAA mimics (indolacetic acid) which duplicate plant hormones responsible for cell elongation which is what leads to the distortion. Thing is, if it was a very low rate of drift I would expect more cupping of the leaves and leaf elongation. At higher rates, the leaves become small and stunted but the stem becomes dramatically twisted.
gmac, I don't have any other pics right now but almost all the leaves on the plant are cupped. That was the first sign of damage. I thought they might have had either too much or too little water. I know it's not too little so I thought they were over watered but the herbicide theory fits much better.
I like the response though. Very thorough.