Bolting is one problem, most likely due to incomplete training (six weeks is not nearly enough time). The other problem, of tipping the cart, makes me think that the pony has gone through the roadside ditch or uneven ground and gotten out of control.
Training must include desensitization to the cart shafts being uneven, one higher than the other, such as would happen if one wheel was lower than the other, or the wheels bouncing through a ditch or rut in the road.
Uneven ground puts pull/twist on the belly band and all the harness, and is undoubtedly very uncomfortable, just like going downhill can be odd. The pony probably didn't know what was grabbing him and over-reacted and tipped the cart.
I would say that if the pony is now frightened of the cart, you should start over with the training, for sure, going very slow to make sure all these "holes" in the training get addressed - whoa, rating speed, uneven ground, and desensitization to the cart.