I read somewhere that if you eat alot of carbohydrates, it
releases some chemicals that can make you feel sleepy.
Also, I think another reason may be the decreased blood flow to the
brain. When you eat alot, the body immeadiately starts pumping
blood to the stomach area (so as to "absorb" all those nutrients).
Decreased blood flow to the brain means DECREASED OXYGEN levels.
Hence, you may feel drowsy, start yawning, fall asleep with food
still in your mouth, etc.
My personal theory, anyway.