It all started in 2012, when Roberts sided with the left in a 5-4 ruling that declared Obamacare constitutional.
More recently, Roberts has kept mum as the court blocked Wisconsin's voter ID law and stopped enforcement of parts of Texas' abortion clinic restrictions. Conservative justices Antonin Scalia, Clarence Thomas and Samuel Alito all made public dissents on those cases.
Roberts' court also declined to hear seven cases on gay marriage this month, effectively letting same-sex marriage stand in those states without having the court weigh in on the national debate. Four justices have to agree for a case to be heard, and it is widely believed that Scalia, Thomas and Alito would have voted to hear the cases.
"I think there are a lot of conservatives who feel like, instead of calling the balls and strikes, he’s kind of ducking when possible," Carrie Severino of the conservative Judicial Crisis Network told Politico.
Conservatives also fear they are seeing a pattern, as they did when justices Sandra Day O’Connor and John Paul Stevens, both appointed as conservatives, slowly moved to the left during their tenures.
"It's about 50-50 that a Republican judge or justice will remain anything like a Republican," Curt Levey of the right-leaning Committee for Justice told Politico.
From Roberts' perspective, he appears to be wanting to avoid any appearance that his court is being affected by the increasingly partisan mood of the country. Has recently made public statements that he doesn't want that partisan divide affecting the court.