It seems that recent master (2.7) implements an auto-straighten when switching on the crop and rotate module. It has surprise me a few times, but it generally is spot-on. Kudos to the dev who implement this; very nice!
Oh dear! I do hope it’s possible to turn that off by default. (Winogrand would have hated it!)
I may have imagined it … in which case it won’t be a problem