I have now heard from one other philosopher who had a trade book (not about transgender issues) rejected from OUP after consultation “with my editorial and marketing colleagues.” Despite “the manuscript [being] readable, accessible, lively, and interesting in the way that a book for a non-academic readership needs to be,” the worry was about the “controversy and sensitivity.” 

The only relevant difference here was that the professor did not have a contract before submitting the manuscript, unlike Byrne.  But the letter does sound similar to the one Professor Byrne received, except the "colleagues" consulted explicitly included those in "marketing" as well as others on the editorial side.  This professor's book was also picked up by another major academic press. 

I should have permission to release more information about this case in a couple of months.   But a worrying pattern–starting with this incident in 2019–is emerging here.