It is hard to tell from your picture exactly what is going on, but it sure looks like you are losing steps badly. There are 3 main machine reasons this can happen.
1) your belt tension is not correct. Be sure all belts are "Guitar string tight"
2) Your Pulleys are slipping on the motor shaft. Be sure the set screws are tight
3) The voltage setting for the motors on the GRBL board are not correct. Search the forum for instructions on how to set them.
You can also loose steps if you are trying to cut too deep or move too fast. Keep your depth of cut to about half the diameter of your bit and keep the feed rate around 40 ipm (at least till you figure out what is going wrong)