He said in that one lengthy interview that he's been incredibly busy shooting a number of feature films, doing press junkets, etc. They talked about reducing his role, and he said "what's the point?" essentially.

Maybe there was a little tension because that's just a tough scenario to manage-- and certainly Miller seemed to resent how Alec Berg treated him during this time-- but if Miller's heart is in doing all these feature things, and the show doesn't want to work its way around his schedule, then it's really for the best all around, probably.

Here's the Miller interview that I'm gleaning this info from:

HR: How did you manage to leave the show mid-run? Didn't you have a contract that would keep you on the series?

Miller: They came to me and said, "Look, we're not going to pick up your contingency because we want to offer you doing five episodes out of the 10, or three episodes." And then I said, "Oh perfect, I had been wanting to ask if you guys would be open to me leaving the show." And then they suddenly said, "Wait, no, what? You can do whatever. What? What do you mean?" And that was so good of them. They said, "We just wanted you to have more time to do all of the things you're doing." And I said, "Well, the best way for me to be involved in the show is by no longer being on it." I swear to God, that's why the internet broke. Everybody was like, "What the f— are you talking about? You're on this successful show. Don't you want three more years of solid acting work and don't you want to be a famous television actor?" And I was like, "No, not really." I'd like to parasail into the Cannes Film Festival for The Emoji Movie because that's the next new funny thing that will make people laugh.

HR: Why were the producers going to reduce your role in the first place?

Miller: Because they had to move the production schedule around. That's how heavy-duty my schedule is. Even the most successful comedy next to Veep on HBO was like this thing that I had to — I'm doing stand-up and I come back and I didn't sleep at all. I was incredibly busy. People joke about it but I'm the hardest-working man in show business, maybe. So they were like, "Let's make this easier for both of us." And I was like, "I think this is an amazing opportunity."