Absent a union contract or other established rule, you don’t have to use seniority to decide which employee should be laid off. You can use any objective measure.
Recent case: David was the only black mechanic who worked at an airport cargo-handling service. He was also the most senior.
David was terminated in an economic reduction in force after his boss picked him instead of one of the other white mechanics with less seniority.
He sued, alleging race discrimination and claimed one of his white co-workers should have been selected. The employer explained it picked David because he had been written up three times, once for smoking in a truck filled with gasoline.
The court dismissed the case. It ruled that employers can use disciplinary history in layoffs. (Walton v. Evergreen, No. 10-2403, ED PA, 2012)