Why Did NASA's Head Of Human Spaceflight Resign Just Before Historic Launch?

Last Monday NASA announced that Doug Loverro, head of human spaceflight at NASA was resigning only 6 months after starting the job, and just over a week before the historic return to flight for the human spaceflight program. We know it's not related to the Commercial Crew Program and is going to have no effect on the upcoming launch, but we don't have much in the way of concrete information to go on.
Right now the theory is that this is related to the procurement of the Human landing system, but we could be wrong.

Update:
An article in the Wall Street Journal has confirmed that NASA’s Inspector General is investigating information shared with Boeing as part of the Human Landing System proposal - confirming my speculation.
https://www.wsj.com/articles/nasa-investigating-former-officials-contacts-with-boeing-on-lunar-contracts-11591538265

Видео Why Did NASA's Head Of Human Spaceflight Resign Just Before Historic Launch? канала Scott Manley