Yes, this sounds imposible, but if you have a scaled model that has been rotated manually so scene XYZ axis are coherent with object principal directions, if you drag the view, your axis will rotate about Z. This unexpected behaviour has driven me crazy for quite a while... and is totally nonsense from my point of view.
As well, it is annoying that if one rotates object, then sets a scale bar and updates... Photoscan will on its own modify model rotations instaid of just scale it
Why without my permission? WHY?
EDIT: This odd behaviour can be reproduced, however it seems not to be a general case. In my particular case, I'm processing photos that had geotags. I cleared them before alignment, but still the coordinate system remains as wgs84. I have noticed that resetting transform and changing to "local coordinates" makes things work as they should.
Regards