I'll make a guess. Trump looked at the polls, and started getting overconfident. So, he thought he could turn a general election weakness into an attack on Clinton. We have also seen another return of Trumpiness.
This is the kind of thing that got Trump into trouble in the past. He has stayed away from these moments for the last month or so, and risen in the polls. Maybe it's a coincidence, maybe not. I've been speculating on that for the last few days. But, Trump himself is inviting the press to start talking about birtherism again while making comments about how Clinton should go without armed secret service protection so that we can see what happens. Maybe Trump is slipping back into Trumpiness. There are limits to his self-control.
Sorry, I have no social science for you today. I have no explanations. All I have is the rough guess that Trump has gotten overconfident from his recent rise in the polls, made a strategic blunder on birtherism, and started letting his Trump flag fly again. This year is just nuts.