Hexagram 15 has a property no other hexagram in the book has, and it is frequently noted: every one of its six line texts is favourable. There is no warning line, no line of regret, no top line where the condition overreaches and turns. In a text whose characteristic move is to show how any good thing goes wrong at its extreme, that is a striking exception.
The traditional explanation is thematic. Modesty, or Humility, is the one disposition that cannot be overdone, because excess of it would be a contradiction in terms. Whether the compilers reasoned that way is unknowable, but the observation about the lines is a fact about the text rather than an interpretation of it, and it is worth knowing.
The name Qian, unrelated to the Qian of hexagram 1, and written with a different character, is rendered Modesty by Wilhelm, Humility by Legge, and more concretely by later translators, some of whom connect it to a word for a mole or a burrowing creature and read the hexagram as being about keeping low rather than about a virtue at all. That reading belongs to the school that treats the Zhou Yi as a Bronze Age omen collection, and it is a serious minority position among translators rather than a curiosity.
The image given in the Ten Wings is a mountain inside the earth: the highest thing, positioned below the surface. Whatever one thinks of the commentary as history, that is a very good image, and it carries the hexagram's argument more efficiently than any of the English names for it do.