Tafsir of Al-Burooj 85:6

( إذ هم عليها قعود ) — "When they were sitting over it" acts as an adverbial modifier for "the killers" [mentioned in the preceding verse]. That is to say: They were cursed at the moment they surrounded the fire, sitting around it in a place near to it, overlooking it from the edges of the trench, as in the saying of Al-A'sha:

It is kindled for two shivering ones warming themselves by it, And a door [is opened] to the fire of generosity and the shaving [of the head].

It is said that the speech contains an elliptical genitive, meaning "at its edges" or the like, but the majority hold that this is the intended meaning without requiring any ellipsis.