Who knows? But the language that he began to be a “mighty one” links up with the description of the Nephilim as “mighty.”
Did he access pre-Flood knowledge?
Did he engage in an act of cannibalism on some Nephilim remains? See this article:
https://www.newscientist.com/article/dn18172-gene-change-in-cannibals-reveals-evolution-in-action/
The issue with this thought (Nimrod) was that he was the son of Cush, not Canaan.
Also, Genesis 6:4 seems pretty clear. Could be there was a second violation.
But Cush was of the line of Ham. If Ham’s wife’s line was corrupt, then you would have some genetic offspring of Nephilim through the line of Ham, which also might explain why Canaan was cursed.