What sin did he perform that requires him to bring a Korban?
Rashi and Rashbam: After becoming Tamei in one of the ways specified in the Pasuk, he either ate
Kodshim or entered the Beis-ha'Mikdash.
Ramban, Moshav Zekenim:
Since it is obvious that there is no Isur in merely touching Tum'ah or (in Pasuk 1) merely forgetting the Shevu'ah.
Which be'Meizid, is subject to Kares (Rashi and Rashbam).
Rashi: He forgot that he is Tamei.