Is there a particular reason why Slade hurts Robin's arm so much?

Im currently rewatching the original Teen Titans cartoon, and i realized that Slade twists, punches, and overall hurts Robin's arm in almost every direct fight they've had, im too lazy too search for every moment but there are enough for me to notice it as frequent, even in the "Haunted" episode (S3 E5) the illusion Robin saw twisted his arm behind his back cracking it.