The 1,100 workers downed tools after Mitie announced plans to make a shop steward and two health and safety stewards redundant after winning a £200m FM contract on the Cumbria site.
Under the deal Mitie is providing building maintenance and security services on the site and employs around 450 staff.
John Fallows, regional spokesman for Unite, told local newspaper the Evening Mail: “Mitie had a selection process that we couldn’t agree with.
We put forward our side, to incorporate certain things, and they wouldn’t have any of it. Everybody else on the site found that alarming.
A Mitie spokesman said the company would not be making any comment at this stage as it was assessing the situation.