He had been retired for four years. This is not a part of Supreme Court history.
July 18, 2016: Obama Administration petitions the 4-4 deadlocked Supreme Court for rehearing on its attempt to put into executive order an immigration reform which Republicans had formulated and supported but then rejected when it was time for a vote. The petition pointed out the necessity of decision on this festering issue and asked for rehearing as soon as a ninth justice is seated. This was four months after Merrick Garland was nominated. As often, Obama was assuming that Republicans would be responsible adults. Or maybe he knew the petition was doomed and was creating a paper trail for posterity.
He taught Con Law at the most prestigious law school in his home state, so I assume he knew more about the issue than some people here.