Computer favors passing trumps, but making his nil is more important for him. Suppose computer has Q, 10 of hearts and A, K, J, 5 of spades. If he passes 3 spades, he will be left with 2 high hearts, and in many cases you will not be able to cover those. But if he passes Q of hearts and A, K of spades, these cards will make sure you can cover him.