From what I've read so far, it's a bit like milling, the jet prefers to cut one way rather than the other, I'm trying to find the post again because I can't remember if it was clockwise around the outside or anti clockwise, easy to test though.

its connected with the way the plasma swirls as it exits the nozzle.

once the first part of the cut passes a bed slat, it's pretty much tack welded down so the contact should be good.