I can relate to what your supervisor does through many interactions between family, coworkers, and supervisors. It sounds like:

"Gunnysacking"
Filed under: English, Colloquial
Part of speech: n.
Quotation: Gunnysacking is a term for storing up hurt feelings, anger, affronts and unresolved conflicts, and, when the weight of the psychological gunnysack becomes too heavy to bear, unloading it, often to an inappropriate degree in an inappropriate context.
From: The Double-Tongued Dictionary at www.doubletongued.org

It all builds up and then when one thing happens, all the anger and resentment from the past grievances come out too and making the current situation much more explosive than it should be. Or then use this information against you in some fashion even though they are personal grievances/feelings and not facts. It is important to learn to address things when they happen than to let them fester. It is a good tool to have.

Conflict resolution is a helpful set of tools for the workforce and in personal relationships:
http://www.mindtools.com/pages/article/newLDR_81.htm

It is not easy when others do this, but it is possible to manage and also recognize when doing this ourselves. Perhaps this might help.