Owen apparently was not expecting such a cowardly blow, and hence must have been taken unawares.

The consequence was that the blow landed on the side of his head when he tried instinctively to duck.

It sounded horribly suggestive, and made Hugh's blood fairly boil as anger swept over him in a wild wave.
Owen staggered and fell.

Gamely, he attempted to scramble to his knees, and before Nick could prevent him had even done this, trying to strike back in return.

The boy was furious because of having been dealt such a foul blow; he would have leaped at the giant just then if the necessity arose.
Nick was in his element.