I'm throwing this out as just a possibility. I don't know for sure that it is "legit". It could be just hearsay.
I've heard that some sports/activities are joint-loosening and, therefore, are not a good mix with pitching. For example, I've heard swimming tends to loosen the shoulders. I've also heard that pitchers who thow opposite-handed from how they bat (e.g. a right handed pitcher who bats lefty) are more likely to be injured than pitchers who throw and bat with the same hand. The reason is swinging the bat is a loosening activity for the front shoulder which is your pitching shoulder if you pitch opposite from what you bat. It would seem backhanded swings with a tennis racket might be similar to swing a bat with the opposite hand in terms of opening up the shoulder.