Maybe set the category of this post to Trading or F&O or NIFTY and Bank NIFTY discussions,
so that the relevant folks who are following that category will see the post and can respond to this.
Also, some of the messages in the following thread-topic may provide some clarity…
Option buying that you see in the last hour/30 mins on the expiry can be broadly done with the following reasons:
Pure option buying as a lottery tick (Many do it with an intention of buying 1-2 rs option n sell it at 100 times profit). Unfortunate if the amount is big, but it is what it is.
Option sellers buying back their previously sold option and closing their positions. (As there is a risk of sudden movement in premiums on some days, it is a prudent step IMO)
Some traders do ultra quick scalping as the price movement of the options is pretty quick during the last hour but yeah, eventually, all the OTMs end at zero and even ATMs expire lower.