How does overeating make you tired?

Probably: Can divert circulation and metabolism from parts of the body to your digestive system thus slowing metabolism elsewhere and probably leading to fatigue as a result.

Overeating: After you eat, the blood supply to your gut increases to help secrete enzymes and carry the digested and absorbed nutrients around. With overeating it requires longer periods of digestion. Since the amount of blood in the body is fixed, this diversion of blood from other organs can make you feel tired.