I can still remember this discussion from my initial ground school for the DC-3.
You are correct about the design of the wing being the cause.
If you look at the airplane from overhead (or underneath) it is indeed a swept wing airplane. If you recall where the stall begins on a swept wing aircraft then you'll see why the DC-3 likes to try to roll over on you during a full stall.
Our company prohibited us from doing full stalls in the airplane, and after having been through one during a training flight with a new hire, I know why!
