Despite bickering all the time, jockeying for position and generally rarely seeing eye to eye both Richelieu and Treville don't take kindly to someone else attacking either of them.
A musketeer badmouthing Richelieu when Treville's in ear-shot? A courtier implying Treville isn't good at his job? They will get shut down epically. Although this never happens when both Treville and Richelieu are present.