The mono's all seem to punch well above their weight class on tough, hard bodied critters like hogs.
Of course, for clean quick kills shot placement is key. It looks to me from the photo like maybe a high shoulder shot right on or above the point of the shoulder which would put it right in the spine for an instant lights-out response.
I've killed a ton of big boars, that's a very nice boar.
guess that's an old wound on the outside point of the shoulder.
I'd guess still it was close enough to the spine for "spinal shock". Pretty well impossible to drop a big hog dead in the tracks without brain/spine disruption.