Blocked water
Actually, there wasn't anything blocking the water--but I swapped in the five-level tower rack with the bellows/freefloating tower configuration. It's not as good at hitting the far corners as the space needle, but you get loads more room in the bottom rack when you don't have the cutout for the tower.
Sigh.
To be fair, this was a filthy load of dishes--and there wasn't any redeposition on the dishes, just this gak layer on the top of the door.