looks like a slight wobble.
looking at the machine - no belts all screws - I'd have to say it's a function of the screwthreads and at least one of the threaded rods isn't exactly straight, or perfectly lined up.

Screw drives are all well and good, but don't have any of the built in give or tolearance, that a belt driven system has.

Either that or the threads are out of sync with at least one of the stepper motors.
that said - it's a cool pattern :-)
But I'd bet on it being something to do with the threaded rods.