Reason.com has an interesting post up today that provides, if not a full background, at least an interesting connection and a hint. The post -- The First Anti-Ted Kennedy Tea Party: Boston's Anti-Busing Brigades --includes news footage from 1974 of an anti-busing protest in Boston.
These protests were in response to the legal rulings that schools shouldn't be segregated. These are white people angry that they will now have to interact with black people.
They use the tea bag metaphor too.