I am pretty sure it is not the wire or longline. Spins appear to be perfectly aerodynamic in my experience. As for winching, the higher the downwash the patient is travelling through, the more likely the spin. The Black Hawk was easy to see, there was considerable tendancy for spin at very high winches (120ft plus), then reduced tendencey as you got lower until an increase again near the door. I always wondered about this until I rode the winch a couple of times and found that the Black Hawk seemd to have quite a large null (like eye of the cyclone) in it's downwash pattern when less than 100 or so feet. That null can be felt on the ground after getting winched down, everything seems fine, a bit of light turbulance is all, then you unhook and go to walk out from under the aircraft and in a couple of paces you are in a hurricane of downwash propelling you away from the aircraft.
I then tested this against the Huey derivatives and found a similar thing until I got to the 412. That did not seem to have much of a calm area to play with, but I dont have much time winching on these. The BK117 on the other hand, has almost no calm area and almost always tries to spin the winch load at some point. But remember, any wind will shift the downwash downwind, so there are times when the downwash effect on the winch load is minimal until near the top. Adn that is why some small forward speed can sometimes help stabilise the spin, because it effectively moves the load just out of the downwash area.
Assymetric loads also spin more. If you are winching with a bag, try and put it between your legs rather that out to one side. As for stokes litters, I would suggest that they NEVER be attempted without a tag line unless the aircraft has very little downwash effect.
As for external loads (shape aside), the longer the line, the less the tendancy to spi in the hover: again a downwash factor. But forward flight is the real danger for spinning, and there are many loads that will spin up to enormous RPM once you get them up to forward speeds, regardless of what length line they are on. A simple drogue line or chute acting as a tail sorts this out easily.
Happy gyrations.