“I think it’s unnecessary in deciding cases to ask that many questions, and I don’t think it’s helpful, I think we should listen to lawyers who are arguing their cases, and I think we should allow the advocates to advocate.”
Most of the general public view oral arguments as the totality of the work that the justices do, but in reality a lot of the work goes on behind the scenes. Reading up on prior decisions, parsing the grammar of a law, reviewing history at the time the law was created, debating behind the scenes, etc. So I could see why asking questions during oral arguments isn't particularly important to him. The other justices do plenty of questioning anyway, so it often becomes redundant.
The second reason he has stated is that he is self conscious about the way he talks because he grew up speaking Geechee (aka Gullah) in rural Georgia. Since he was teased about this dialect, he formed a habit of not speaking.