United Airlines launched earlier this year MileagePlus promo called Mile Play trying to get select members to spend and fly more. They have had these offers every few months and just launch a new one for December.
These promotions are tailored to each member based on his/her recent activity with United. It may require buying more business class tickets or flying certain number of trips before the end of January.
You can try your luck registering for this offer here.
United Airlines has run similar offers for years and launched these “Mile Play” promos in early 2018. Seems United decided to gamify this offer more based on the name used.
My account was not eligible for this (never has been) because I moved it to outside of North America few years ago.