If an entity is in a boat and is using a tool with the knockback enchantment, when it hits the player, the player will be flung in only one direction no matter which side the player walks towards the boat.

How to re-create:

spawn a mob that can use weapons (such as a zombie) and place it in a boat, then give the mob a knockback item (level does not matter), you will notice if the player (while in survival mode) walks towards the entity from the back the mob will knock the player towards itself rather than away, if you move to the side of the entity it will knock you left/right rather than knocking you backwards away from itself as it seems it is only able to knock you in one direction.