There are many reasons and sometimes there are no reasons at all, other than the producer doesn't like that name and he has the right (according to most model releases) to name the girls whatever they want. The reasons can be distinguish the girls, for example if a studio already has a model with the same or similar name they might want another name. Also it's a trick to reintroduce the girl as new model, however on the long run usually it doesn't work but could be good for a few times. And sometimes the models themselves decide to use different names, sometimes for the same reason: to sell themselves as new models or try to escape from a breached contract or their old agent

But I agree it's annoying sometimes and not too good career step either.