Teamwork issues and being a leader = more work
A couple of rants:
Grad school focuses more on group work than tests. I believe that is at least partially due to the fact that professors want fewer papers to grade, but I'll leave that for another rant. So most classes entail a group project due at the end of the term that counts for a significant portion of the grade. Coming in to MIT, I had a delusion that group work in grad school would be different than group work in the workplace. I'm sad to report it is not. I'm part of a group for each of my four classes. Each one has had a non-trivial amount of "teamwork issues" that has caused more stress and delays than anything else. Lots of reasons for this ranging from people having different objectives and motivations for a class to the classic teamwork issue of people just not getting along.
Second, I've found that being a leader in these groups often leads to more work for the leader. The person that usually voices his opinion about something and tries to set direction for how to proceed has to begin with taking some part of the assignment. If you have 4 or 5 people in a group, the last 1 or 2 people to speak up often have very little to do. Or through the course of a discussion, people will generally agree to take some part of the assignment. But those that don't voice their opinion much, also don't volunteer for stuff. I'm not exactly sure how work would get done in these groups if they didn't have any leaders, but I guess that is true of most situations.

