This is going to be mechanical on the Y axis (up/down). Normally when LightBurn runs an engraving, it moves to the bottom, reverses, and engraves bottom up. When it reverses, if there is slack in the belts, it’ll take a while for that slack to be taken up, and the laser won’t actually start moving the other way until that belt slack is gone.
It could also be a loose coupling or a set screw that’s loose on one of the Y-axis motor pulleys, but look through those parts - it’ll definitely be something in there.