As stated in Wikipedia (link: http://en.wikipedia.org/wiki/Ford_sewing_machinists_strike_of_1968#cite_note-wainwright-4):
The Dagenham sewing machinists walked out when, as part of a regrading exercise, they were informed that their jobs were graded in Category B (less skilled production jobs), instead of Category C (more skilled production jobs), and that they would be paid 15% less than the full B rate received by men.
This would be a good lead in to a discussion of unions or discrimination. One question to consider: if markets are competitive, would you expect people of the same skill to be paid less? What would we expect to happen over time?