This porcupine is showing it's vulnerable underbelly. It is one of the few places on the critter that predators can claw or bite without being pierced by hundreds of sharp quills.
On the ground, porky high gear is not very fast. When threatened, porcupines usually put their back to the danger, and sometimes whip their tail around. Domestic dogs seem to be the only animal that doesn't know better than to move boldly in on the thirty thousand quills that one porcupine is armed with. Dogs are are also known to go back for revenge on the porky before the painful quill-pulling-party hangover has worn off.
Those quills are barbed, so they continue to work in deeper if they are not removed. Since the quills have an air pocket inside, they are easier to pull out if the end is clipped off first.
Our new buddy has decided to take the slow way down.