That's a loss of steps. Either something mechanical is blocking the movement of your X/Y axes, or you don't have your stepper drivers properly calibrated.
The stepper drivers have a small trimpot on them that needs to be set correctly. Too low and you lose steps, too high and things overheat. There may be a better procedure, but I do mine by trial and error. I usually start my turning then all the way down (CCW) and then up by about 1/8 turn - and then experiment from there.
