The answer would be that the human being is a whole whether indivdually treated with CBT or in group (mindfulness is a type of CBT). In anxiety norepinehrine turnover in nucleus coeruleus is increased and in depression 5-Ht level in nucleus raphe is changed and so on. THis is an oversimplication but depression does not have a specific spot on the HPA-axis, the human mind affects many transmitters and when the mood lifts the level of transmitters are effected in many parts of the body.
Helen Mayberg gave a fantastic talk at Society for Neuroscience that drew upon multiple sources on how CBT may help with anterior Cingular functioning . Granted her talk was on her deep brain stimulation technique. But there was evidence from multiple sources.
I can't attest to monoamine changes specifically. I think it's broader than that (as
Hofmann, S. G., Fang, A., & Gutner, C. A. (2014). Cognitive enhancers for the treatment of anxiety disorders. Restorative Neurology And Neuroscience, 32(1), 183-195.