It’s a combination of hair pulling on wax and exacerbated by the salt water. I get chafing on my inner/rear thighs, armpits, and upper belly. Going to try compression swim shorts under board shorts next summer. I need to surf several days in a row multiple times a day for it to force me to take a day or two off. Also if you use a deck pad instead of wax that is more abrasive. I have one on one of my boards and it’s pretty rough even with a rashguard. Vaseline helps a little if you’re already raw but still want to surf. There are companies that sell board shorts with built in liners to help with leg chafing (driftline.co)