It explains it in the court rulings....  

dThe norm is that the company has become accustome to a certain amount of overtime/workovers being conducted each pay period/month.   Just like the airlines do for pilot picking up Open Time.   They build it into the schedule.   it is predictible.   It is expected.   There is an economic norm established.

So, now you send the company official negotiation letters and enter into bargaining, and at the same time, CHANGE the norm to put pressure on the company.   That is a work action when you should be operating STATUS QUO (no changes).  You have union pilots calling for a "no overtime/workover" effort in order to put pressure on the company at the table.   It was openly stated.   They are attempting to call it "voluntary", but hey have established a normal to what "voluntary" means.   A simple statistical analysis from HR/Payroll can easily show a correlation to Section 6 vs the amount of overtime/workover that is being covered.   If there is a "norm" established, any deviation from that "norm" is easily correlated to a hush-hush economic workaction.   

That why the courts ordered them to cease and decist, and ordered the Union to denounce it offically and control their (militant) members from acting in a wildcat fashion.   It is ILLEGAL under the RLA to take action against your company until released by the NMB to enter self-help.   And, the quickest way for that to happen is for either of the two parties to enlist the mediation services of the NMB.   Seems the company did just that while the union complained they did.   Seems the company is seeking to come to a conclusiong in negotiation than the union, judging by that action.

BTW, seeking NMB mediation is generally a normal part of the process in negotiations, and is not considered to be bypassing it, as the union accused them of doing.