Lol.... strangest R&P question yet.
Dunno what films you mean, but it could be just a dramatic device that the directors are using to make some point. (And probably a negative one, I shouldn't wonder). As in, the cropping of the nun's hair representing the Catholic Church's suppression of individuality or female sexuality, or whatever.
(Mind you, I've heard that in girls' orphanages run by the Sisters of Mercy, the nuns sometimes cut the hair of girls who tried to run away. They did this, they'd say, because they thought these girls were vain, and wouldn't try to venture into public again until their hair grew back.
Of course, the girls were more than likely trying to get back to their families, or to escape the frequent abuse of these institutions. But it seems from that, at least, that the Church equated a girl's hair with her vanity and/or sexuality.)