* The monster's effect prevents it: Some monsters have effects that specifically state they cannot be changed to defense position, often during certain phases of the game or under specific conditions.
* An effect from a card on the field: There are several cards, both monster and spell/trap, that can prevent monsters from being changed to defense position, either temporarily or permanently.
* A card effect that changes the monster's position: Some cards force a monster to change its battle position, either to attack or defense. If a monster is forced into attack position, it can't be switched to defense.
It's crucial to read the text of all cards involved to understand any restrictions on changing battle positions.
Here are some examples of cards that prevent changing to defense position:
* Monster: "Cyber Dragon" (cannot be changed to defense position if it has no other monsters on the field)
* Spell/Trap: "Battle Fader" (prevents monsters from changing battle position during the Battle Phase)
Remember: The game rules don't have an inherent rule that stops you from switching a monster to defense position. It's always a possibility, but certain cards or monster effects might prevent it.