I'm going to take a WAG here and say:
Perhaps the pilot group felt the offer was a little light on remuneration or working conditions, or;
Perhaps what was offered in exchange for the stock options was less than the worth of the options effectively meaning a pay cut rather than a status quo or even a raise.
Perhaps both.
The article posted only had one viewpoint, that of the company spokesperson. In fact, the Headline is seriously biased. I very much doubt the pilot group voted against expansion, they voted against the agreement they were offered.