On what basis do you make that assumption?
I think he was compromised. Eric Holder runs the FBI and I don't think John Roberts is free from secrets. Supreme Court justices are just as fallible as the rest of us and I believe that Obama and his Chicago minions had the goods on Roberts. If in fact this decision was extorted out of him, then Obama and company probably included an early resignation as part of the deal to bury whatever they had on him.
If Roberts resigns in September, look for HRC to be appointed to replace him. And if HRC is appointed, don't expect any fillibuster on the nomination from McConnell.
Obama and his minions play dirty. They are as corrupt as Al Capone ever dreamed of being. The only thing that adequately explains Roberts decision is that Obama and Holder had him over a barrel.