This is a pretty graphic video.
The poster wonders why they keep on fighting, and I think the answer is simple:
This was filmed in February, which is the mating season for both species. I’d say the male skunk is the primary aggressor, and he’s spent weeks fighting other male skunks in the area. He’s been fighting and mating all month, and he’s just so full of testosterone that he can’t help but attack the opossum, which sort of gives off the vibe of being another male skunk.
The male opossum has been doing much the same, so when he’s attacked, he keeps coming back for more.