How does Riot choose the championship skin for the season?

I always thought Riot pick champions that have had a major impact in the season for instance Thresh and Janna the previous few seasons. How exactly does Riot pick which champion should they make the championship skin for in a season?

The reason i ask is cause I thought this might be championship MOrgana , cause she has been a revelation this season as a strong support and found her back to competitive play and then...Riot decides championship Shyvana , who has not had a lot of impact this season.

Any thought on this?