Brahma Sutras – According to Shankara 2-3-8
Topic 8 - Reabsorption takes place in the inverse order to that of creation
विपर्ययेण तु क्रमोऽतः, उपपद्यते च ॥ १४ ॥
viparyayeṇa tu kramo'taḥ, upapadyate ca || 14 ||
viparyayeṇa—In the reverse order; tu—indeed; kramaḥ—order; ataḥ—from that (the order of creation); upapadyate—is reasonable; ca—and.
14. (At Pralaya the elements are) indeed (withdrawn into Brahman) in the reverse order from that (of creation); and this is reasonable.
The question is whether at the time of cosmic dissolution the elements get withdrawn into Brahman in the order of creation, or In the reverse order.
The Sutra says that it is in the reverse order, for the effect goes back to the causal state, as ice, for instance, melts into water.
Hence each thing is withdrawn into its immediate cause and so on in the reverse order, till Ākāśa is reached, which in turn gets merged in Brahman.