I have a Villager trading hall underground, and when I go up to a Villager and click on them to trade with them, I automatically start eating the cooked chicken in my left hand. I continue to eat the chicken until I have completely eaten one, to which it stops. If I exit the trade screen before eating a chicken, the eating stops.

I do not know if this only happens once or will happen over and over depending on hunger level, but I suspect the game sees me as clicking and holding, and since I typically have a tool in my right hand, my left hand item is used, likely providing it is a consumable.

This happens with potions as well, and, upon further testing, a consumable item in my right hand is used as well. I quickly entered and exited a Villager's trade screen with a Swiftness potion in my right hand and after the second exit the potion was consumed.