Sparks are caused when steel strikes rock. Pigs like to wallow in mud, especially in hot weather, and in the great wild world we call nature, sometimes small rocks get mixed into mud. I’d guess your pig had some dried mud on his body, and your broadhead (steel) hit a small rock, making a spark.
Outdoor Skills – Hunting Q&A
"I recently went on an archery hog hunt in Texas with several friends. When it was still pretty dark I shot at a hog, striking the neck. The arrow made a spark when it impacted. The pig ran off with my arrow, leaving only a blood trail. I couldn't find the arrow or the hog. My so-called friends claim the broadhead couldn't have caused a spark and laugh loudly when I insist it did. They no longer want me to hunt hogs with them, saying I'll start a forest fire with my sparking arrows. Do you know