Check the bottoms of your Z threaded rods. If they're not flat, and the uneven edge is in contact with the motor shaft, artifacts like this can occur. You can just file them flat or better yet, file them flat and then drop a BB into the tube so it provides a single point of contact on the bottom of the screw.

Another thing to check is to be sure your filament is unrolling freely into the extruder. I saw something similar to this once when my filament was tangled and it spent the night pulling against a half-hitch tied around the spool.

Someone also suggested making sure your hot end wasn't loose, and I didn't see you respond to that, specifically. You said your axes were all tight, but is the hot end tight in the carriage? Is there any wiggle in the way the hot end is mounted to the extruder?