The following contains major spoilers for the Demon Slayer: Kimetsu no Yaiba manga by Koyoharu Gotouge, available in English through Viz Media.
Tanjiro and Nezuko Kamado's lives were forever changed when Muzan Kibutsuji, the King of Demons, slaughtered their family and left Nezuko to become a demon. As Demon Slayer plays out, it's clear that Muzan's goal is to become the strongest demon by overcoming his weakness to sunlight, one of the few weaknesses a demon has. What remains unclear for many fans, though, is Muzan's motivations for turning Nezuko into a demon.
To everyone who encounters her, it's clear that Nezuko is no ordinary demon, but did Muzan have any way of knowing she would be such a deviation when he made the decision to turn her? Did his attack have anything to do with the Kamados' ties to Yoriichi Tsukiguni, or was it pure happenstance that tragedy struck the Kamados and Nezuko would eventually become such a formidable opponent?
How Did Nezuko Become a Demon?
Fans who've been reading the Demon Slayer manga might think Nezuko becoming a demon is a result of the connection between the Kamados and Yoriichi Tsugikuni. Yoriichi was the originator of the Breathing Styles used by Demon Slayers and master of the Sun Breathing style. He nearly killed Muzan with the technique, leading the latter to spend years hunting down anyone who'd come in contact with the style to ensure it could never again be used against him.
The manga leads fans to believe there might be a familial connection to Yoriichi, only to show that he had just spent time with the Kamado ancestors during his travels. While the Kamados had no demon slayers in their family, through Tanjiro's memories it's shown how the Hinokami Kagura dance that his father passed down evolved from the basic tenets of the Sun Breathing style. However, Muzan may have tracked them down; it seems possible that his fear of Yoriichi's legacy would drive him to eventually kill the Kamados -- but would not necessarily explain why Nezuko, or any Kamado, would be turned into a demon.
Becoming a demon is no accident and -- outside of Lady Tamayo -- only Muzan has the power capable of creating demons. Because each new demon born is a deliberate act on his part, what made Nezuko stand out as a candidate? So far, the Demon Slayer anime has made no allusions as to a reason, but the answer is ultimately revealed at the end of the manga's "Swordsmith Village Arc," and again in its final arc.
It might've appeared random that Nezuko would be turned into a demon, as she's nothing more than a child in a poor family in the mountains. While such a tragic circumstance could drive her to willingly accept the transformation -- as has been shown by demons like Rui, Daki and Gyutaro -- her becoming a demon is surprisingly coincidental. Muzan simply creates demons in the hopes that one will be created with the power that he seeks: immunity to the sun.
Some demons succeed in surviving and join the ranks of the Twelve Kizuki, but not even the strongest of these have developed the ability that would make Muzan all-powerful were he to devour them. There's no indication that Muzan had any way of knowing whether Nezuko would survive, let alone become the unique demon she is. In the end, Nezuko was nothing more than a random victim in Muzan's search for power. Every demon he creates is essentially an experiment, and every successfully created demon that lacks immunity to the sun is still a failure toward his overall goal.
Nezuko becoming a demon was technically planned, in that she was another one of Muzan's experiments, but the type of demon she would become surprised everyone. Much in the same way that Tanjiro has continued to defy the odds in his journey as a demon slayer through sheer will, Nezuko's will fuels her growth as an extraordinary demon. Muzan's decision to turn her into a demon for his own gain ends up being one of his biggest mistakes.