Start by addressing the number of letters in each word. The words always have an even number of letters, but that isn't very unusual. Six- and eight-letter words constructed in this manner are rare. The word MAIL has this unusual characteristic.

MOMS DEMAND FLORAL MEMORIAL IN MAINLAND GALA
LASCAR INKS MANDARIN COAL MINE DEAL
ARCADE GAME INCOME GAIN ALARMS MAMA
COOK MADE CALAMARI OR MAHI MAHI MEAL
MARINE ARMS LAND MINE IN DEMO