i grew up with bull terriers when i was younger. they are stubborn, if they get something in their heads they are going to do it. imagine the bull terrier ran full speed at the porcupine bit it and realised oh hell rolled over it and ran away= a hellava lotta spines.
our bull terriers ate round a peach tree till it died. ate wing mirrors off cars. chewed up a whole plastic garden chair.
when I was growing up a buddy of mine had a bull terrior. She liked to chew up power cords. he lived in a old house and the breakers wouldn't trip so she just kept getting shocked, it would make her mad so she would tear up the cord even more. I can see how this could happen with a porcipine.