That's just fat. A shield is scar tissue from fighting.
You apparently haven't watched hogs fight. You would see that a large percentage of their attempts to wound their adversary are in the neck and low on the sides via upward thrusts. Strangely, no shield grows in these locations.
Also, if the shield was scar tissue, it is remarkable symmetrical and must have pigs being a bloody mess most of their lives. That just doesn't happen.