Some of them felt Boeing timed the 2014 vote so that many of the Machinists would be on vacation and wouldn't vote (January 3, 2014).
In 2014 the Machinists lost their pensions, they got a 4% wage increase over 8 years, and Boeing shifted health care costs further onto employees. That wasn't a great deal for the union and the members haven't forgotten. They felt like Boeing messed them over when they had the leverage and the opportunity.
Now they feel like they have Boeing over a barrel and they want a lot better deal.