Graduate students unionizing is the norm in public universities. As the article points out, it's only recently (2016) that the NLRB ruled that graduate students at private universities are to be considered employees, which opened the door for students to vote whether they wanted to unionize, prior to this they had no legal standing to even consider unionization.

[0]https://en.wikipedia.org/wiki/Graduate_student_employee_unio...

Not sure I would say Harvard itself is "leading" this. The university was found to have been interfering with the unionization process.