your form is poor and you are doing too much weight is why
most people that do pull downs i SMH
pulldowns should work your back #1. and to work your back FORM is so important. most meathead idiots use their shoulders 95% and that's why their backs are pathetically weak
stop with the ego and engage your back only when you do pulldowns.. pretend there is a penny in the middle of your back and you are trying to squeeze that penny