Tafsir of Al-Qalam 68:19

فَطَافَ عَلَيْهَا طَائِفٌ مِّن رَّ‌بِّكَ (Then, there whirled around it a whirl (of calamity) from your Lord, while they were asleep….68:19). The word 'whirl' refers to 'a whirl of calamity' as a result of which Divine punishment overtook the garden in a sudden and swift sweep. Some narratives report that it was a fire which burnt every standing crop, and reduced it to black ashes.

وَهُمْ نَائِمُونَ (...while they were asleep.... 68:19). In other words, the devastating disaster overtook the entire garden while its owners were asleep.