落 = 艹 (grass radical) + 洛 luò (phonetic — a river name). The original meaning was leaves falling from plants — the grass radical anchors it to the botanical world. Water falling was already embedded in the phonetic 洛 (a river whose name evokes flowing and settling). Together: the natural downward motion of organic things arriving at rest.

This character is the second half of 角落 jiǎoluò — the corner where things settle, the nook that gives this site its name. 落 in that compound contributes not motion but arrival — the state of having come to rest in a particular place. A book-room in the corner is books that have landed there.

落 also appears as a phonetic-semantic pivot across many characters: 骆 luò (camel — the animal of caravans that settle at oases), 洛 Luò (the Luo River, historically crucial), 络 luò (a net; to entwine — things falling and wrapping together).

When 落 follows another element, it often means a settled place, a cluster of things that have landed together. This pattern is especially visible in place names and spatial vocabulary. A 部落 bùluò (tribe, clan) is literally a group that has settled together. A 村落 cūnluò (village) is a place where people have set down roots. An 角落 jiǎoluò (corner) is where things settle at the edge.

The same logic appears in less obvious words: 零落 línglùo (scattered, withered) describes leaves that have fallen and lie scattered; 沦落 lúnluò (to sink into degradation) is a person who has fallen to a lower place. All are variations on the image of something descending and arriving at a new — often diminished — state.