Historically, what factors precipitated the formation of unions? Please explain.

Read the articles:

1) Grad Students Win Right to Unionize in an Ivy League Case

http://www.nytimes.com/2016/08/24/business/graduate-students-clear-hurdle-in-effort-to-form-union.html?_r=0

The New York Times, August 23, 2016

2) Doctors Unionize to Resist the Medical Machine

http://www.nytimes.com/2016/01/10/business/doctors-unionize-to-resist-the-medical-machine.html

By NOAM SCHEIBERJAN. 9, 2016

3) America Needs Labor Unions

http://www.huffingtonpost.com/dale-hansen/america-needs-labor-union_b_6930794.html

by Dale Hunsen - March 24, 2015

4)South Carolina Boeing Workers Reject Machinists Union Representation

http://nlpc.org/2017/03/13/south-carolina-boeing-workers-reject-machinists-union-representation/

by Horowitz, Carl-13 March 2017

5)Go to the internet and find an article that discusses labor union formation and activity, summarize key points

Historically, what factors precipitated the formation of unions? Please explain.