I do believe that a bit of background on Nietzsche's Human, All Too Human would make any interpretation of Thus Spoke Zarathustra richer. Allow me, then, to draw from my knowledge of the said book, aside from so-called authoritative commentaries on Thus Spoke Zarathustra, such as The Mask of Enlightenment by Stanley Rosen.
Rosen used a translation different than yours: "those who do not know how to live, except by going under; for it is they who cross over." As I understand his interpretation, the people who "do now know how to live" are Zarathustra's disciples, hence Zarathustra loves them. You might ask as to why Zarathustra's disciples do not "know how" to live. The answer is that the reason why they came to Zarathustra in the first place, why they became his disciples, is to learn how to live authentically (as an ubermensch).
Now, in your translation, the "down-goers" are the ones who went "down" under Zarathustra's teaching, under the patronage and guidance of Zarathustra. Since they benefit from the teachings of Zarathustra, they are the ones who may or might become the ubermensch of tomorrow, the "over-goers."
While I agree with Rosen, I assert that there can be no single interpretation of Nietzsche, as Gregory Smith already asserted. Nietzsche himself wants to be interpreted multifariously. The reasons for these are:
a)Nietzsche, as some degree, deliberately and consciously wants to be misunderstood;
b) Being misunderstood will call for a closer examination of the work misunderstood;
c) A closer examination will reveal the depth of profundity and the richness of the said work;
d) Nietzsche, in his books, addressed different types of audience, yet believes that an advice to a type of audience may still be applicable to another. He is a bit of a subscriber to the thought that the author "does not matter" anymore after his books are published;
e) Each person brings with him his own "lifeworld" in reading the text and the said "lifeworld" affects the way he reads the text, thus giving rise to valid, different interpretations.
Now, in my own reading of that passage, I can recall Nietzsche's teaching in Human, All too Human that when one becomes ill or burdened or "fettered" in some way, he/she will compensate for it in another way. Nietzsche's own example is the blinds who develop a superb sense of hearing. When hearing is concerned, they then are superior to the rest of mankind.
The same, I believe, is applicable to the "down-goers." Though in this case, "down-goer" must mean someone who is in the stage of the camel, extremely burdened, who, as a way of compensation, will increase his courage and will come later to throw off all that burdens him (lion) then create new values for himself (child.) The child is, obviously, the over-goer, for he went "over" that which afflicted him before.