I understand your point. In the announcement over at Reddit, people have noted how much of a textbook PR move this is looking like, so I'm assuming the "this is why" is equally calculated and not the whole picture.
However, you're right. The two reasons are not mutually exclusive.
Why don't you believe her when she says it was a disagreement with the board over the future of reddit?
"We don't know" when the person in question is saying "this is why" is not very respectful.