What format are you batch rendering in? If it is a high quality one then maybe you are running out of virtual memory (i dont really know what virtual memory is, but I know that batch renering uses it up and if you have it set to close a program that uses up all the virtual memory then it will just stop) I had this happen to me and couldn't find out what the problem was either, I think batch rendering is something that will be perfected in maya 7.