One explanation could be that your data is still in slant range geometry. What seems like a square in your SLC product actually covers a long stripe due to different pixel resolutions in azimuth and range direction.
Have a look at these sources: https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/ers/instruments/sar/applications/radar-courses/content-2/-/asset_publisher/qIBc6NYRXfnG/content/radar-course-2-slant-range-ground-range
