I quote from my notes when I did the type rating in 1992...........the aft/bulk and fwd cargo doors are an intergral part of the a/c structure..........towing the a/c (particularly in the case of tight turns) with these doors open can subject the a/c to tortional stresses which are beyond the initial design capability.
That said I have seen the a/c towed with doors 1 L and R open and cannot explain why this is also not mentioned in the notes.