The whole "seniority gives you the right to work, not the right to not work" argument is only applicable if a sup. asks an hourly to do something, and he says, "No, I have more seniority, ask him (hourly with less seniority) instead .
This is trying to use your seniority to refuse work, which you cannot do.
If cuts are made, like in the OP's situation, they have to be made from the top FIRST. Supervisors don't ask lower seniority workers anything before they've asked those with top seniority. Vacations, leaving early, OT, double-shifting, leaving the area, preferred jobs - it's all covered by seniority.
It actually says in the contract, "Seniority will be given prime consideration in all work related issues."
The company does try to use a loophole here and there, usually after peak, where an hourly who has yet to make probation, isn't considered a Union employee, and thus, the company doesn't have to abide by the rules of seniority when making cuts. But I've always told the offending supervisor or lead, if that's the case, what is he/she doing touching packages?
They usually roll their eyes and ask if I want to go home.