Based pm what I'm seeing you're experiencing tool pull out. If you measure the tool length with a ruler before and after use it will most likely be different lengths. This is why several other users and I have moved to the NR-2551 spindle. You can see it here
https://www.youtube.com/watch?v=FAh6ktwH8SI. Once the tool has been pulled out it will be milling deeper than what the program things, thus gouging parts. The pull out occurred in in the picture with the horizontal indent marks. You can get around this otherwise by reducing your feed rate or reducing the depth of cut.
Aaron