Thanks to chenistry and his/her post on r/churning I got offered 40k bonus miles when I spend $2000. Click here to see if you qualify, it looks like its targeted to those with any United Premier Status. I am currently silver and got this offer:
I asked my normie non status friend Davey from TrainWreckTrader.com and he verified that he got a easier deal than myself:
Looks like in all cases those with higher offers spent and booked major flights the day/week before, not sure if this triggered the invite, but looks like United is trying to capture our spend for the second remaining half of 2016. Best of luck out there!