وَإِذَا ٱلنُّجُومُ ٱنكَدَرَتْ
Root: ك-د-ر. The concrete sense: to grow turbid, to lose one's clearness. Kadir — turbid; its opposite ṣāfī — clear. In Arabic it is used of water growing muddy: kadira'l-māʾu.
From this comes the metaphor: kadar — the clouding of one's peace, the darkening of the heart. English has no word descended from this root; Turkish took the word over directly, and there keder means grief.
ٱنْكَدَرَ — Form VII (muṭāwaʿa): "it grew turbid, it clouded over of itself". In Arabic this pattern conveys that something happens of its own accord.
1. It grew turbid, its light went, it darkened. 2. It poured down, scattered, fell. Inkadarati'n-nujūm — the stars scattered. Also inkadara'ṭ-ṭāʾir — the bird swooped down, came flocking.
How do the two stand together? For a star, "to grow turbid" and "to fall" come to the same thing: whether it stays in place and goes out, or whether it falls, it has vanished from the sky. What the observer sees is the same result. The word names the result seen by the eye, not the physical mechanism.
The Qurʾān's parallel: "When the stars are scattered" (al-Infiṭār 82/2: wa idhā'l-kawākibu'ntatharat). There intithār (scattering) is used — which supports the sense of falling.
Of the sūra's twelve verbs this one alone is not passive. The sun is rolled up (someone rolls it); the stars simply cloud over (of themselves).
This is a point about the word order and it is worth recording. As a possible reading — my own reading: when the source of the light is taken away, the lights go out of themselves. An operation is applied to the sun; the falling of the stars comes as the result of that operation. The difference of pattern lodges the relation of cause and effect inside the grammar itself.
I do not present this as a certain intention. The verb's coming on the Form VII pattern may also have to do with the harmony of the verse-endings.