This article here: FBI working with artifact collector to return items says the 91-year old contacted the FBI --- "Northern said Miller had contacted the FBI about returning the items, but couldn't elaborate on why Miller was looking to repatriate the artifacts now."
Here's my own semi-educated guess, for what it's worth: I'll speculate that when he collected these, they were all legal transactions, as far as he knew.
But since that time, Political Correctness has mandated that all things Indian have been "stolen" by the evil White Man. Which is why Native American artifact collections have become illegal and must be "repatriated" immediately to the government or a museum or whatever, and all sports team names (eg Redskins) that have been stolen from them must be immediately returned, etc etc etc.
So my first guess is that over recent years he has watched, horrified, in the news how his decades of loving labor have changed from being his pride and joy in a lifelong accomplishment, to instead being an ex post facto shameful, evil life of crime (or at least in the eyes of our cultural masters).
Also, another speculation: I'm wondering if he saw an internet article or TV documentary on items that were stolen decades ago from other private collectors, recognizes many of them as ones he bought from someone posing as the legal owner, and so just now realizes that he was the unwitting "fence" for those stolen goods a half century or so ago.