For me it has been and always will be when a management team hires people that just don't really care to do any better day by day, and refuses to cut dead weight when it becomes apparent that the employee is just not willing to raise their standards to the advantage of both themselves and the business. When management allows mediocrity to be the defining characteristic of the restaurant's culture it forces those employees that actually care to pick up the slack constantly, ultimately leading to burnout and resentment, which inevitably breeds toxicity.