Have had this happen to me quite a few times. Often there is no reaction at all and the hog runs off to fall dead some time later. Recently shot a 130 pound sow low behind the ear with a .22 magnum at about 50 yards: Knew that i had hit her but there was no reaction at all. She took off with her big pigs like nothing happened. There was no blood or anything. Started walking in ever increasing semi circles and found her dead about 90 yards from where she was shot.

Not sure why it happens.