Burley uses a polymer hitch (the black plastic) which allows some degree of movement between the trailer and bike:
Most inexpensive trailers use a spring to provide that flexibility:
I've only used the cheap spring hitches which can easily absorb the twists involved when the bike falls over or the trailer flips (don't ask). I wonder what would have happened with a Burley. It seems the polymer would have broken. Are the Burley hitches stronger than they seem and do they have advantages over the simpler spring design?