Well let's see, cabin crew were the only group to vote against the Greenfield plan. We could suppose that it was because of a certain militancy amongst Cabin Crew. More realistically they were probably lead to believe by the union that a better deal could be obtained.
The Union actually endorsed accepting the package at the time as they had negotiated it with the company. The original plan by management was to basically save €20m by freezing pay/ cutting allowances/ removing grades. The union managed to find an alternative way to save that €20m, increasing productivity. The crew rejected it as it came hot on the heels of the previous cost cutting package of €15m, and indeed hot on the heels of the previous 3 or 4 cost cutting rounds. Crew were fed up with a management team constantly on the take and not introducing the promised benefits, eg computerized rosters which took YEARS to finally replace the handwritten rosters published every 2 weeks.