There is a million possible reasons why the professor and the university do not work together anymore. Some of these reasons might be private matters which should not be shared with the public. Tenured appointments aren't broken easily, so you can assume it's either one very serious problem or a huge number of small problems. The problem might or might not be relevant to you personally. But unless you have good reason to believe that the problem is relevant to you, acting on the assumption that it is would be unfair.
You could always ask, but if the professor doesn't seem to be willing (or able) to speak about it, pondering the issue will just hurt your relationship. But if you really want to find out what happened, you might want to ask people in person, not per mail. It might be a matter nobody wants to make any statements about in writing.