Miller was the one who asked for his release, according to Dave Meltzer on Wrestling Observer Radio.
Miller has a newborn baby back in Australia so he wanted to be home near his family. He decided that he did not want to return to the United States right now and that is why he requested to be released. In addition to that, TM61 wasn’t in the middle of a push and there were no signs that they were going to get a push anytime soon which probably made his decision easier.
There are no bad feelings between him and WWE and he was told that the door is open for him to return.
NXT UK wrestler Tucker and 7-foot Chinese prospect Taishan Dong are also gone from the company. We don’t know the circumstances around Tucker’s departure but the word is that Dong requested his release. He signed with the company in July and had been training at the Performance Center.