if each layer starts in the +y direction, and the machine always rapid travels in +y direction to begin each layer, and if the machine regularly misses y step(s) during the +y rapid travel, then the layer dimension will remain correct (as long as it isn't missing steps at feedrate), but each layer will be shifted in the -y direction by however many steps were missed during +y rapid travel
first, make sure the toolpath looks correct, and then reduce the rapid speed and acceleration, and if that helps then it suggests that you were missing steps when the part curved