Let's say you get a mass-email from your boss containing some organizational information. You have a question, so you send a reply to your boss (not everyone) asking for clarification.
When your boss receives this, the "technical" default behavior of any e-mail client in the world would be to only send the response to the person who sent the e-mail. However, for obvious reasons, your boss may want to include everyone that got the original mass-email, in case someone else has the same question.
Now, for the question of etiquette:
Should your boss:
- Reply to the e-mail, then add everyone back to the conversation, and reply?
- Reply to the original sent e-mail, writing a more general addition with the answer contained?
I am a strong believer that #1 is very bad form, as it leaks an e-mail thread (your question) that you don't have consent to spread on. In most cases it's probably harmless, but it irks me nonetheless.
There are of course way worse examples of "e-mail thread leakage", but this one seems pretty straight forward, and is clearly well meant.