cr:7.4.2d

When a defense reaction card resolves it becomes a defending card on the active chain link for its controller’s hero.[7.0.5] A defense reaction card fails to resolve if it cannot become a defending card.[5.3.2b]

Example: If an attack has dominate (can’t be defended by more than 1 card from hand) and there are two defense reactions on the stack, the first one will resolve and become a defending card, but the second one will fail to resolve because dominate prevents it from becoming a defending card.