Of course it's mechanical but so is bat drag. I don't think a light glove is a 'cause' but maybe she wouldn't throw the arm out so far if the glove had more weight. I think that pitchers swing the arm out as a way to get leverage to help close the shoulders, something they shouldn't do.
One of my 16U students got a heavier 'better' glove in mid season and told me that the extra weight threw her off. She practiced with it on and off for several weeks before adjusting to it and using it in games.
Ohhh interesting, Im in the boat that the glove is too heavy for my first year 10u dd. I think she's trying to move her glove arm as fast as her throwing arm but cant because there is 2.5x the weight on the glove arm. So she uses her shoulder to move her arm faster.
I could see the opposite happening at older ages. Where a heavy glove might aid.
Or I could be totally wrong!