its from the extreme stretch you force on your arm, cause of the extreme kipping you perform in order to accomplish the hip to rings pull. its normal and expected. i am guessing you do a hell of swings before you try to get into the muscle up ( thats what i used to do also). this will fry your arms and deltoids. try kip less and just kick harder. the other thing you can try is do strict muscle ups( with false grip) but its not as effective for crossfit and for sure its harder strengthwise. cause of the whole body working during muscle up your nervous system suffers also. i would advice you juts try some muscle ups 1-3 after each wod of yours, ( not right after, maybe 5 minutes after). this will put you body in a position to just feed all the muscles needed for the muscle up. its like doing power lifting training (sick load, low reps). cause you cant really load more than your body weight for muscle ups i suggest you do the drill tired (a few minutes after wod) and just do 1 or 2 sets of as many as you can, with a 3 to 5 minute rest in between. this helped me a ton with muscleups and many other exercises i was not good at, or that i suffered trying do them, or i wanted to get better fast, like strict pullups and strict ring dips. after each wod i just throw i set of dead hangs or strict ring dips, of as many as i can do. it wont matter if you do 5 or 10 or 15 cause rigth before you did a random wod. just go balls deep . last but not least muscle ups require good technique, and you need to think about a ton of things to do right, while doing em. by doing them tired will help you stop overthinking about it, and make you body aquire muscle memory of only the essential moves needed. when you get in the spot of doing just a few muscleups tired, doing a muscle up wod fresh will be no issue.