Presumably, it was a much ruder interruption to the class to stop everything and publicly humiliate a student whom he felt disrespected by. And it's extremely rude to take a personal email and forward it to the entire class (who presumably would be able to recognize the student).
Maybe the intent was to "smack some sense" into a student for their own benefit, but that's really presuming a lot about a person from a handful of data points.
Also, how likely is this to be BS?