I think the suggestion of the belts being different either end, (could be swapped around) or the rollers not parallel, having the same effect, or try moving the bobbin (stability roller) the the bale is going towards, in to the middle a bit. Before someone invented those things, if you had a bale that started walking, it would go on walking till it fell off. Getting it central in the first place was crucial. Possibly also running the thing too fast.
Spoke to a very helpful man at conors today, says it will be the belt on the left hand side that's probably needing replaced, l may can ttightn it a bit if not 4 new belts. Try the tightening options first.