I'm not saying that there's no such thing as a racquet that's too heavy for anyone, but more often than not, that shoulder fatigue comes from using a frame that's significantly heavier (in either way) than the one that you're accustomed to. That heavier racquet takes just slightly longer to get going to the contact point. If your ingrained timing doesn't get the frame to the ball on time, it's the shoulder that's usually called upon to rush it to the ball in a sort of "catch-up" maneuver. A quick service motion that calls upon too much work from the arm and shoulder will usually generate this fatigue faster than anything else.
Either a frame with higher static weight or significantly higher swing-weight can wear down a shoulder. In both cases, the racquet needs a slightly earlier move to hit the ball on time. I try to encourage any players that want to sample heavier racquets to do it on the practice courts where they can primarily concentrate on the tempo of their strokes. In a match where we're trying to focus on the points, our opponents, etc., we swing at more balls unconsciously and that's where more of that painful rushing of the racquet can happen.
Bad technique will obviously wear on a shoulder, too, but if a racquet is heavier, it needs different timing for a comfortable swing. Even if the shoulder doesn't fatigue right away, a player might get soreness in their wrist or maybe some golfer's elbow from a late push of the racquet to the ball.
