Not stepper motors, but maybe a loose coupling or belt.
You have backlash in one or both of your axis.
When cutting a circle each axis has to change direction during the cut.
If there is any backlash on an axis screw, that axis stops moving while the screw/belt/rack catches up to the backlash during changes of direction, hence the straight cut.
This can also be caused by an axis hitting the soft limit and causing a flat on one side only unless it happens to be a really big circle which could make more flats.