There are more hammers in town.

No, it's inventory is just full. Since the first closed beta build of Hamlet, these guys would spawn in new hammers to wield every now and then. Primarily when logging in and out of the game. That is why many times you can kill a single one of these pigs and get over 20 hammers. At this point however, I assume his inventory is too full, so he simply drops the hammer. (Much like Moleworms/Moleworm Burrows with too many items.)

Next update will clean out the pigs' inventory, and will add a console command to clean the ones laying around in the city if you choose so.