I hunt hogs alot. Here in west central Florida we have them by the bunches. Anyway, though I haven't lost one yet, (This due to the cartridge/projectile combination I use more than intrinsic skill...)I can speak about their incredibly tough constitution with some authority. I shot my biggest to date through his cheek and into his body. The 180gr Swift A-Frame smashed one half of his thorasic vertabrae for about 8 inches. In other words he was paralyzed on one side. Thinking he was dead, I laid my Weatherby 30/06 to one side, and walked over to him.
Fortunately I approached him from his rear, for when I got within tusking distance he lashed out with those 2.5" tusks. I had forgotten the classic African hunter's axiom: It's the dead ones that kill you. Anyway, to make along story short, I delivered a coup-de-gras with a 22. Gratefully I didn't end up with a new set of hard to explain ankle scars to show off.
So to loop back to the topic at hand: It sounds as if you did everything in a responsible manner; sometimes we do everything we can think of and we still come up empty handed. Feral hogs are tough as nails, they can absorb a substantial ammount of punishment and still dish out some themselves. My guess is that your shot went high passsing close to the spine, and temporarily stunning the nerves in the front half of the body. After the shock wore off the pig hi-tailed it to parts unknown. If it missed vitals, it probably recovered. Pigs are creatures of habit, keep an eye out from that stand and you just might bump into him again.
PS Hogs are addictive! If this should happen to you, please grap your smokepole, pyrodex, and favorite projectile, and run to the nearest safe hunting area. Do not stop to talk to neighbors, this only increases your anxiety!
Try this forum: www.huntamerica.com
Check out the hog forum.