Mass firings are seen as a opportunity to get rid of your "worst" employees because you have strong plausible deniability if the reason for firing is anything other then the massive layoff. Employee trying to start a union? Raising politically sensitive issues? Under-performing? Questioning management? Negative personality? Taking too many days off? Not popular on your team? Companies are aware of this and it contributes strongly to the negative signal assigned to be involved in a mass layoff.