Reading the sentencing document it appears that committing an "unintentional second-degree murder while committing a felony", appears to be deserving of 12 years in prison. However, given that:

(i) That Mr. Chauvin abused a position of trust and authority;
(ii) That Mr. Chauvin treated George Floyd with particular cruelty;
(iii) That children were present during the commission of the offense; and
(iv) That Mr. Chauvin committed the crime as a group with the active participation of three other individuals, former Minneapolis Police Officers Thou Thao, Thomas Lane, and J. Alexander Kueng, who all actively participated with Mr. Chauvin in the crime in various ways.

He was judged to deserve an extra 10 years in prison.

He still faces the federal charges from the incident, and 9 felonies related to his and his ex-wife's tax evasion.