Lawyer up if you need too. The do need to provide you with the cause, not just, "an external force." Tell the dealer and BMWNA that you will be speaking to your lawyer about a lemon buy back and that you need all of the vehicle's documents, (completed work orders, maint, etc.)
Ard, (member on here.) I believe just lemoned an X5 and can probably give you some more advise. But the laws do vary from state to state.