the doubling in percentage suggests students are becoming more informal...or that students are communicating more via email with their instructors. And classroom etiquette is a broad category. Writer doesn't mention etiquette regrading communication with professors.. Etiquette could involve interactions with professors and other students in class.
In fact, when I clicked on the link none of the etiquette issues in the survey pertained to email communications with professors:
"While just 14 percent of syllabi in 2004 addressed classroom etiquette, 33 percent did so in 2010. Here again, anticipatory socialization was apparent: “I expect you to bring the same good manners and concern for others to this class as you would to a work or professional encounter [B10].” Some prescribed behaviors such as “going to the bathroom . . . before class” and “scheduling . . . appointments outside of class [A19],” while others proscribed behaviors like “sleeping, talking, eating . . . [doing] Sudoku puzzles [B15].” One instructor insisted on “decent normal human behavioral expectations,” explaining that “shuffling papers and preparing to leave before the end of class will be considered culturally rude and unacceptable [A9]!”