When Yukino realized that Yui had asked naruto as well, she was willing to back out, in short giving her a clear shot at him (she was surprised as well, meaning she didn't know that Yui had asked him). As for the rest, just the whole drama of the situation and a lot of looks between the two.
But had a thought. In the first episode Yukino said "because I am cute all the boys liked me and that caused me a lot of problems with the girls". So perhaps "sacrificed" is not the right word. Maybe she was afraid that naruto would fall in love with her and this would drive Yui away, which Yukino does not want to happen.
This would tie into what I said about Haruno discounting Yui. Because in this series, when it comes down to it, Yui has had the bigger impact on Yukino. Having guys like her is common, but having a girl honestly like her was, to Yukino, something new. This is interesting because it was to thank Yui that Yukino had originally asked naruto's help and Haruno saw them together (and became focused on naruto). This isn't to suggest that this is a YuixYukino ship (though that would be funny), but rather that it is Yui's friendship and example that is what is changing Yukino.
It would be ironic, both Haruno and Hayato have focused in on naruto, and indeed have become a bit obsessed with him (because I think to both he is the key to their salvation), but if I am right, then it was Yui, the girl both discounted, who paved the way.