it would appear your are not domiciled in Utah for 2016 (based on 2018 instructions)
you may be a resident (full year or part year) however:
even though domiciled (permanent residence) outside Utah, you maintained a place of abode (where were you living while attending college) in Utah and probably spends 183 or more days of the taxable year in Utah. When determining whether a person spends 183 or more days in Utah, a "day" means a day in which the person spends more time in Utah than in any other state. (i'm putting it this way because you say you filed as non-resident which would require a permanent place of abode outside Utah)
Part-Year Resident Defined
A part-year resident is a person who is a Utah resident for part of the year and a nonresident for part of the year. All income received during the period of Utah residency is taxable in Utah, regardless of where that income is earned, unless specifically exempted. Income from Utah sources is taxable in Utah during the period of nonresidency.
you would probably be considered a part -year resident not a non-resident
you needed to file form TC-40B (for part-year and non residents) along with your Utah return. That way your California income would only be taxed there and you Utah income would only be taxed there
Utah, based on whatever info they had, regarded you as a full year resident.
The notice probably contains a contact number which you should call to get advice on how to correct the situation.
In the worse case scenario, Utah says "no" you a full year resident, they would have to grant credit in whole or part for the taxes paid to Ca
you may have an additional problem, since it seems you had a permanent place of abode outside Utah, if it had an individual income tax for 2016 and depending on the tax laws there, you may have been required to file at least a part year return (you say you move to Ca permanently in December)
