Good question (because I don't really know the answer!)
This is a very common form of words, so it's perhaps a distraction for us to look too much to this particular quote. Here are some (made up) examples of the same kind of thing:
One of the biggest problems in Australia is the rabbits, rabbits that are not indigenous to the country.
All was quiet until the soldiers came, soldiers who had run away from the front and who were hungry, afraid, and desperate.
The core of stackexchange is the questions, questions that confuse, intrigue, and beguile those who take the time to read them.
I think the use of the definite article for the first instance of the object in question -- rabbits, soldiers, questions -- is fairly standard. In each case we have a specific plural noun. There is only one relevant set of rabbits, soldiers, or questions (in each case, all of them). I'm not sure as to the reason the article is omitted for the second instance, but my guess is that we are no longer denoting a particular (plural) object, but rather are merely providing additional information about the already-denoted object (as opposed to providing extra info to allow us to distinguish from several possible objects).
That effect can be seen if we see the sentences as having a silent "which are" or "being" just before the second mention of the object.
Sigh; that's the best I have. But I'd wait for a better answer if I were you.