If I had to guess, he knows that when things turn back around, he wants any and all reasons for people to be on his side. Keep a close eye on his decisions post 2020.
There is definitely a reason. Supreme Court Justices have policy preferences like anyone else, and if a particular decision would hurt the electoral chances of their preferred politicians then that's a reason to go the other way.
Whether or not they actually do this is another question, but the incentive is still there even with a lifetime appointment.