Wow, compelling question. I thought a lot about this and think that in this instance, the idea of the "shadow persona" really does help understand.
(What leads to the shadow persona to manifest I'm not sure; i.e. if it is spiritual, psychological, neurological, or some combination.)
If some of the qualities of the "shadow persona" were present but less detectable/prevalent, it makes sense that these could become exaggerated to the point of distortion during MLC.
For example, someone who couched criticisms in gentle ribbing (like my husband) could suddenly reveal these to be deep grievances and resentments of the shadow. Whatever once balanced out the shadow seems to disappear.
Just thinking through, but I'm rereading a book on emotional health that discusses the importance of early childhood in laying the foundation for an emotionally healthy person. If, as we often see in the stories on the site, a MLCer does not have this, the social mask her or she developed to balance the shadow may be quite flimsy or be disguised by professional success, accomplishments, and the structure they end up in by midlife.
My feeling is that the traits we see are an unchecked "shadow persona." This can include traits that were nonexistent, subtle or even undetectable prior to MLC, as well as more obvious but harmless-seeming traits. If the LBS is observant, however, we can also see that some of what the shadow persona expresses is positive and creative, and this is highlighted during MLC as well.