The answer is "the relevant facts have changed or the problem has been resolved by other means."
Mootness emerges when there is never again a genuine debate between the gatherings to a court case, and any decision by the court would have no real, practical affect. In the event that it is resolved that all issues for a situation being heard in a U.S. government court have turned out to be disputable, at that point the court must expel the case.