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Answers in Genesis and Creation Ministries International, two young-earth creationist organizations, have similar answers to this question. They don't attribute a direct supernatural cause, but instead attribute the age drop-off primarily to genetics.
Both cite several factors:
Some of the ideas that both reject as insignificant are a change in diet (from vegetarian to omnivorous) and reduced oxygen levels after the flood.
Regarding Genesis 6:3, see this related question. AIG interprets the verse to mean that God was allowing mankind to live another 120 years before destroying it with a flood:
We can see that Genesis 6:3 does not deal with the length of time a human will live. Rather, it indicates the longsuffering nature of God in the midst of His wrath. He was allowing even the most evil people of Noah’s time 120 years to repent of their sin before he would judge them with a global flood.
References:
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There are several ways we can find a biblical basis for the reduction in lifespans, at different levels from the literal to the literary.
At least in part, a biblical literal explanation is in Genesis 6:3, where God ordained that henceforth humans will not live beyond 120 years:
Genesis 6:3 (NIV): Then the LORD said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.”
A likely literary example can be found by comparing the lifespan of Adam, the first person in the Book of Genesis, with that of Joseph, the last person in the Book of Genesis. Adam is portrayed as living for 930 years, which is (30 x 30 + 30). Joseph is portrayed as living for 110 years, which is (10 x 10 + 10). The ancients were fascinated with numerology, and it could be argued that the biblical genealogies are literary examples of this.
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An alternative (or additional) answer would be that the age before the flood was counted in seasons, which would correspond with Seth being born when Adam was around 33 and Enosh when Seth was 26.
It's still gives 200+ lifespan before the flood but makes the gap between the two periods much narrower.
Disclaimer: the suggestions comes from "Genesis. A New Light On The Origins Of Man, the Earth, and the Universe" by fr. Guido Bortoluzzi.