Here is one idea. When you force people to change there password you interrupt their workflow. Person X wants to do thing Y but first they must change their password. Now they are annoyed and are still wanting to do thing Y so they try to do the simplest thing possible. This will probably be using the same password again, and when that (hopefully) doesn't work they will choose a another common password all so they don't have to spend more time thinking about it. Now with what the university has done the person is only slightly annoyed and the workflow is not interrupted, this means the person has time to think up a good password with out time pressure on them so they are more likely to come up with good passwords.