Is there evidence of suppression of the frontal cortex during herding behavior?
This is probably not a complete answer to your question.

There have been a few studies on this relationship.

This article from the Journal of Neuroscience "Downregulation of the Posterior Medial Frontal Cortex Prevents Social Conformity" (Klucharev et al. 2011) approach the research (from their abstract):


  Here we demonstrate that the transient downregulation of the posterior medial frontal cortex by theta-burst transcranial magnetic stimulation reduces conformity, as indicated by reduced conformal adjustments in line with group opinion.


This article may also be of interest