It's called postprandial somnolence, and it's not perfectly understood, partly because there's a lot of stuff potentially going on. We know there's a big shift with eating from the "fight or flight" sympathetic nervous system to the "rest and digest" parasympathetic nervous system, and this shift means overall low energy and a drive to reduce muscle use and send blood to the gut instead. There are probably a bunch of other shifts in the brain that contribute, though. Interestingly, chemicals like histamine and orexin that regulate appetite also strongly regulate sleep (i.e. blocking histamine makes you sleepy and hungry.)