An enumerator for a set will start by generating some item, then another item, and so on, in a way that every element of the set will eventually be listed. If the set is empty, then it won't even generate the first item. If the set is non-empty but finite, then it will eventually generate the last item and stop.
What does it mean to enumerate a set in decreasing order? You would start with an existing enumerator that defines some order. And whenever that enumerator generates A followed by B, the enumerator "in decreasing order" will generate B followed by A.
If the set is empty, then "decreasing order" is just an empty enumerator. If the set is non-empty but finite, then "decreasing order" means we enumerate the last element in the original enumerator first, then the element that was enumerated before the last element, until the first element of the original enumerator is generated, and then the enumerator "in decreasing order" stops.
But what if the set is infinite? The new enumerator must generate some set element A first. The original enumerator would generate A followed by some element B. In decreasing order, B would have to precede A, but it can't because there cannot be anything before the first item. For example, you can't generate the primes in descending order.
So if a generator can enumerate a language in reverse order, this means that the language must have been finite in the first place. And every finite language is decidable. Infinite languages cannot be enumerated in reverse order. But that doesn't say anything about being decidable or not.