I had this same thing happen with that particular element in the design! I thought I was going crazy. I couldn't find anything to attribute this to and wouldn't you know it, this always happened when I walked away from the machine for a few minutes. There was nothing near the arm or front of the machine to interfer with movement of the hoop and arm.
I contacted Hoopsisters to see if anyone else had the same problem and they said no one had encountered this.
I did stop using the resume function. I re-loaded the design at the start of every session from my USB stick.
To finish my project I sat in front of the machine for the whole stitch out of each block, camera at the ready to take a video of the strange stitchout, and this never happened again.
I have since used my 15000 for all sorts of other designs from different sources and have had no other problems.