Typically, it is more important for management to assign blame for a problem than it is to actually fix it.
Your sup lacks the authority to solve the underlying problems that are the root cause of misloads (excessive workload, overcrowding, understaffing, poor planning etc.) So instead, he bullies you into signing a piece of paper taking the blame for something you may not have had anything to do with. He can then show that piece of paper to his boss and take credit for "doing something" about the problem.
I would suggest you avoid signing confessions in the future. They serve no purpose other than to provide your management with ammunition to use against you.